Hi Carl
The TRF have a group for adventure bikes here
https://www.facebook.com/groups/advbiketrailriding/Your regional groups are listed here ;
http://www.trf.org.uk/find-your-regional-group/I am not familiar with Northhants for big bikes- although I am scheduled to be led form Bicester - Silverstone - Warwick in October - date to be arranged.
If you are new to riding on unsealed roads, you may wish to come to a Herts TRF run, including at this time of year any beginners run.
Trevor Judd - Herts TRF- does an excellent bike run from Hemel Hempstead heading into Ashridge & Chesham You can contact him via Herts TRF open group,
https://www.facebook.com/groups/HertsTRF/ . I do big bike runs on Salisbury Plain & Thetford forest.
You should look at adding /keeping an eye on Herst, Cambs and Oxford TRF groups and pages.
Finally - you could also consider doing this run on 27th September
https://www.facebook.com/events/256637554845473/ which should be very suitable.
Hope this is helpful.
Regards
Mario