I've just spent a weeks holiday there - took Murphy and stayed at a place called little lodge farm - littlelodgefarm.co.uk I think they let you park the lorry's there - fabulous riding and rivers to cross - even a XC!
On there, there is a "the brecks visitor guide" which has a general map which includes bridleways.
There's also a leaflet called "Riding in the Brecks" which you have to order (£2). I think you can also pick this up at some of the tourist offices in the area.