Thanks for the suggestions. I did the ride today and went by Nicky line, then Grand union canal, then Wendover arm of the canal to Wendover then by road to Longwick then Thame then followed National cycle route 57 to Oxford from there. Spotted on the way was a cycle route that goes from Princess Risboughrough to Thame and if I do it again I would pickup at Longwick which would have avoided the only really unpleasant bit, the Thame road. It was my first really long ride and took 4hrs 20mins moving time to cover 87km with a lunch break of half an hour.
For anyone planning similar routes i just found the Sustrans site which highlights what would be cycle friendly roads and tracks on their map. but doesnt include the towpath bits.