If its cross country head north towards Daventry, and turn left at Pryers Marston and there are some good trails around Hellidon, Oxfordshire/Warwickshire/Northamptonshire border.
That would be great, Normally if possible we head of somewhere else but sometimes we only get the chance to ride together in a very short time allowance and has to be Banbury where he lives.
If you pop a few junctions down the M40 there is plenty in the Chilterns around Stockenchurch but I suppose that a trip down the Mway does not fit into the 'popping out for an hour' ride formula.
Wattlington, Chinnor etc. is great. Watch out for a trails marked CR6 and CR7. Fantastic downhills near Chinnor, that come out on the Ridgeway near a village called crowell. Wattlington area's great too, Christmas Common is worth exploring.