I know theres Wood Farm which is DIY and Sunnyside which is ful livery only.... they are pretty close to Ampthill though not sure how close to newport pagnall that is?
What about MK Eventing Centre? Not sure if this is close enough to you (Hanslope) It look's fabulous! If i lived there I would deffo keep my boy there. Fab place ran by lovely people
What about Lower Farm Stables in lavendon,( they have a web site). Very good yard, won an excellence award from the BHS a couple of years ago. Proffesionally run by a very close friend of mine too.