First, a big thanks to all the mumsnetters on all the great advice you offered. We're now back from Centerparcs (Elveden) so a not-so-brief summary, in case it's helpful (especially, maybe to Galaxy leaving soon!)
Background: we went for one week (Monday-Monday) staying in a three bedroom executive villa - four adults (me, DH, DD (aged almost 5) and DH's parents).
Overall: Great holiday - even with so-so weather. I would DEFINITELY go again.
Villa: We were in No. 953, a newer built villa near the country club/spa. It was very comfy - three nice sized bedrooms (two double and one with 2 twin beds), each bedroom with its own small television, wardrobe and bathroom including tub - one with jacuzzi. Decent sized living room with two couches, coffee tables, tv and telephone, and a small fireplace. Good sized kitchen well equipped with full oven/stove, microwave, dishwasher and enough plates/bowls/flatware/pots/utensils to suit basic cooking/eating. A dining area (living room, kitchen and dining area all open-plan) with a table and six chairs. A small patio with a plastic table and six chairs, and a small outdoor sauna. We had daily maid service, and towels were changed three times in the week, and sheets twice. What we really loved about the villa was that it felt really seculded, even though we were in a row with a whole set of others - it's really well planned. I would stay in this newer area again - it's quite quiet and the cleaning lady said these are about the nicest villas (she strongly advised against the "comfort villas". It was also nice to be near the spa, but it's a pretty long way to the central square and facilities, which made us reluctant to go back and forth too often.
Activities/childcare: there was more to do in a week than we could possibly have taken advantage of. I think this was the reason that none of the facilities felt over crowded, despite the huge number of villas (which seemed pretty well booked). People were spread out all over. We tried a bunch of spa activities - the general spa is great: a series of steam/heat/meditation rooms and a heated outdoor pool with water jets, and I would highly recommend one of the three hour sessions (they were running a special of 16 pounds per person if you had a child booked in one of the children's programs for that time) - but especially in the evening (6-9 pm) when it was a lot quieter. I was less impressed by the treatments, which are quite expensive and should be top-class but were instead kind of mediocre. I had an ok massage, Hopi ear candles (don't bother!), manicure and pedicure. My husband had a massage (better than mine) and a facial which, by his description, was pretty lame. By the way, the manicure/pedicures are in the central village part, not at the spa, and so less "luxe" feeling. I would use the spa again, but probably pass on treatments. We did pre-book some treatments, and it was pretty diffcult to add others while on site - they book really quickly, especially on weekends - so if you want something despite cost/quality factor, try to book it ahead. I also took advantage of a jazz dance class, which was lots of fun and not too difficult. My husband and father in law did the birds of prey - fun and interesting. Our daughter did a few Time Out sessions in the Cricket club - they are 2-3 hours long and have a general theme which drives the crafts and other projects. She liked them, and the facility and staff were quite nice. I felt very confident about leaving her there. She also did a couple of "parent accompanied" activities: Teddy Bear's picnic, Canopy Play and Animal all-sorts, each of which was ok, but pretty short. We also took her twice to the free children's entertainment in the afternoon, which was an interesting puppet show of Cinderella one day, and a balloon animal guy the other. She really enjoyed the main playground, which was a nice facility for little ones, but not too comfy for adults (they need to add some more/better seating!) The much-advertised water facility was nice but smaller than I imagined. Still, we had to tear our daughter away every day - she loved it, and it was not as crowded as I had feared. The lockers and changing rooms are quite well planned, including a series of family changing rooms with extra space so you can all go in together. We went to a film in the small theater one rainy day - a decent selection for all ages/tastes, but seemed the same every day, so if you are a big film goer, it might be too limited.
Bikes: we left renting bikes until pretty late, which was a mistake, because getting around by bike made it a lot easier to navigate the large territory. Since it's pretty flat, this was easy, even if you're not a great cyclist. We rented regular adult bikes, and on the back of mine was an extra children's bike attached (like a full bike, but minus the front wheel, if you can imagine - kind of like a tandem bike, but she had her own pedals/gears, and it bends in the middle for going around corners). That worked really well, and you can rent helmets and buy locks if you don't have your own. There still seemed to be plenty available to rent, even though we did it late, but I think if you wanted to rent one of the buggies to attach to the back (brilliant for smaller children) or a three-wheeler (great for older adults), I think it would have been a problem. Anyway, I wish we had gotten bikes straight away. I don't know what the lines would have been like on day 1, but it was really easy to return on the last day, without any huge lines at all.
Food/shops: restaurants were the one area I think Centerparcs let us down. Other than La Sapiniere, which is a "French" restaurant with a decent menu and good wine list, and the Indian restaurant, which was attractive and quite tasty, the other restaurants had poor service, not great decor, mediocre food and high prices for what you get. We tried most of them and would definitely not go again - I think in hindsight we should have done a bit more self-catering, as it was more pleasant to sit at the villa - especially when the weather let you sit outside. The on-site supermarket was impressive - not very large but very well stocked, and with some prepared foods (like grilled chickens) to take out as well, and a decent bakery section. There's also a large Sainsbury's not far away, and we also brought in a bunch of M&S goodies for cocktail time. I would try to steer clear from most of the restaurants, but book Indian and La Sapiniere to break it up a bit. There is also a fast food type place called Platform 5 which is at least predictably "bad" (hot dogs, chips, jacket potatos) and self serve, so it's quick. The coffee place was also pretty good to get a decent espresso to stay or to go (take it to go and sit by the lake - so much nicer!) Other shops were quite good, including a swim supply store with a bit of everything you might need, and a very nice sweets shop strategically placed in the middle of everything so you can't take your kids anywhere without passing it! There are a decent selection of toilettries/books/magazines and medicines in the grocery store. The big disappoint to us was the toy store, which has an emphasis on tv character "I need one of those" toys rather than things to play with or do something with. Otherwise, we found the general selection of shops quite good and the prices pretty fair.
Other: we used the laundry facility once to do a load of laundry - it's pretty small (four machines, I think) but I only saw one other person there the day I did our load. The load capacity is very large, so what I thought would be two loads was fine for one, and ditto for the drier - you need pound coins to operate - two for the wash and one for the drier, and supposedly, the soap is already in the machine, but this isn't very clear so I added some I brought just in case! They also do service washes for 6 or 9 pounds (depending if you want ironing) to deliver back to your villa, which I would have done had I known. Still, the wash cycle is short - just 30 minutes, as is the drier. A note about the television programs in our villa: there are regular channels and a few Sky channels (movie, news and a CeeBeebies). Despite the literature all saying that there are no DVD/video players, the shops all sell DVDs and videos, so I am guessing a lot of people must bring their own and hook them up!
That's about all I can think of, but am happy to answer any questions to return the favor others have done me!