I LOVE me a good spa day!! Not been to anywhere around there, but here's a trip advisor link where you'll get some candid photos and reviews. I would normally avoid anywhere too big at the weekend as they can be really busy:
I have been to Whittlebury Hall loads of times, but it can be a bit hit and miss. I have had some lovely treatments and one superb 5 course dinner, been to a decent tribute night and the facilities are nice. The only issues I have had is it can be ridiculously over crowded to the point people are lining up to get in the sauna/steam/jacuzzi and obviously this is far from relaxing. It is the mecca for hen dos so can be loud. The rooms have paper thin walls and if you get a noisy neighbour you will hear everything they do, although they are nicely furnished. If you go during the week it should be great.
y spa at wyboston is lovely. You can mooch in the pool with a glass of vino. Whittlebury hall is nice but a bit 'cheesy' (if thats what you want its perfect!) Not many 'rooms' to visit. The centreparcs spa at woburn is fantastic. I have no idea if you can book it separately to a 'stay' but I would imagine so.