GTW,
My first line was a joke
Its refreshing to think of visual navigation failure, usually people have a 'thing' about GPS failure
The Trevor Thom books [nav one I think] contains the required info, or alternatively do an internet search, I once found an excellent site which goes into detail of VOR / NDB tracking [can't remember where though]. Also your instructor would be a vital source of info. There is not much to it at PPL level, I think the only requirement for the GFT is to be able to make a 180° turn on instruments. The rest [VOR / NDB tracking] is carried out during training only I think.
Rgds
EA