Thanks everyone for your comments. Your advice has been so helpful and it meant we did a lot more research before we took on our puppy. Apologies for the long post! But I wanted to let everyone know what we've decided and how it's going.
After this thread we spoke to a dog trainer and a different cocker spaniel breeder and to a friend's dad who is a vet and all of them said that as long as the puppy was healthy they thought it would be fine as long as we train her well. We have already had a puppy trainer come to our house to teach us and will be attending classes after a Christmas. I really love this forum for advice and I don't want you to think that we have done this without doing everything we can to get it right. 
We talked a lot to our friend's parents who were breeding the puppy. They live next door (or rather a few doors away!) from a professional breeder who's been very involved with the puppies. We've also had her checked by our vet who was very pleased with her health. So I think a "backstreet breeder" characterisation isn't fair here. And although we've paid a nominal amount for the puppy, it is not enough to generate any real profit to them.
In terms of socialisation, we're very happy with how much people contact she's had: various local children and the breeders' grandchildren have visited, and of course she's had lots of human attention as the only pup left! We received almost daily photos of the puppy - she was very much loved by the breeders. She was very friendly when meeting us and our children; not at all shy.
Dog-wise, she's obviously had her mum and there's also another grown dog in the house. But she has only had a few encounters so far with other puppies. So I'm expecting potentially some issues there when her vaccinations allow her to meet other dogs properly. We'll just have to see.
So in case you hadn't guessed, we picked her up from the breeder, just over a week ago. She has been a complete star and is now completely house-trained, sleeps all night in her crate and has generally been much easier than we expected (she 'sits' and 'stays' on command and mostly will 'drop it'). She is a very clever dog.
The one area we've had difficulty with is her nipping. I guess this is down to being an only pup: she's not had any other pup nibble her to know how it feels! She particularly goes for the children's slippers or any loose clothes (flapping skirts, dressing gown ties etc). But both the vet and the puppy trainer think there's nothing at all unusual here, so it may be that she's no worse than any other puppy - I don't have any past experience to draw on. She generally doesn't bite hard enough to hurt but does go a bit crazy when I am wearing my bunny slippers. Is this unusual? From the puppy thread it sounds like other puppies nip too.
Just to pick up on a couple of other points that arose in the earlier posts:
- Running: seeing her in the garden, I can see now that this probably won't work! I will still try when she's big enough, but I suspect she'll find the world too interesting to keep up. I know someone who does run with a Springer spaniel but every dog has their own personality.
- "Designer" dogs: Sprockers have been bred for a long time by gamekeepers so I don't think it's right to apply the "designer dog" label to them. Their history's quite interesting, so it's worth a look on Google if you have a spare minute. (My husband put this point in
).
So overall we're really pleased with Dpup and how she's fitting into our family. We're working on her nipping to show her the right way to behave and how to avoid becoming over-excited / too playful - it is definitely worse when she is tired. I can't wait till we can take her out properly when her vaccinations have all kicked in, as we live near lots of green space that she'll love.
If anyone has any tips from their puppies on how to teach them not to nibble people's clothes or shoes, I'd be very grateful too!
It was a really tough decision and I do sometimes panic -especially when this thread resurfaced! She is a bit of a sweetheart though.