lots of good places in milton keynes , check out the parks trusts website
Parks Trust - The Parks Trust - used to work for them so know for a fact that they have loads of good locations.
another four that are worth checking out are
a) the hanson centre / linford lakes which is run by MKC , you have to pay an anual sub (a fiver i think) to get a key for the hides but its a nice site
b) little linford wood just to the north of the ouse, which is run by bbowt (not to be confused with linford wood in mk itself) , this has decent orchids, flutterbys and even doormice tho you'll be lucky to see the latter
c) slightly further afield check out stockgrove country park near leighton buzzard - very good ops on nuthatch here as they are used to people
and finally
d) slighly further afield still check out summerleys nature reserve which is near wollaston , run by northants cc this has an excellent small bird feeding area giving good ops on reed wardbler, tree sparrow, long tailed tit , and ocassionally brambling amonst others