In reply to leland stamper:
Can't help for once you're there, but if you're starting from Bristol, you could stop off for some bouldering or trad in Leicestershire on the way there/back without too much of a detour (or if the weather's bad, the walls in Birmingham, Coventry or Milton Keynes ).
Forest Rock is the pick of the Leicestershire bouldering but it's at it's best in the 7s and 8s. There's a few problems from 5 to 6B, but not much. It also has a handful of routes with a VD, a VS and a good E1 that may interest you. Access is sensitive as it's in the middle of a village, right next to houses, but it's fine if you're quiet and sensible.
For lower grade bouldering, there's plenty of quick drying granite at Cademan Wood and Broad Hill. The average quality there is not great, but there's enough hidden gems to keep you going for a session or two and most of it is low enough to be fine on your own with just one pad (some of it is hilariously lowball).
Or there's some low grade bouldering/micro routes at Beacon Hill.
As other's have mentioned, Slawston Bridge has some surprisingly good vert sandstone bouldering for a bridge and may be close enough to justify a trip from Peterborough itself, rather than on your way there/back.