Fin spar inspection at base technical notice TN107/T50 was issued 08/87 and followed an abused Skylark shedding its fin.
The BGA website summary says that it is annual and after heavy landings, ground loops etc. The written TN as originally issued states that it was a one off requirement and then mandatory after a heavy landing or ground loop. I've checked this with the VGC technical officer who says that the written TN is applicable and not the BGA summary.
The TN demanded the removal of a sizeable chunk of ply at the base of the fin to allow inspection and it would have been a major annual effort. The fact that the original ispection has been carried out (in 1987/88) should be recorded in the glider logbook.
Having said that, given the present day availability of fibre optic borescopes a peer into the area under the fin via a small hole is however a very good idea as part of the CofA.
P.S. I can rig a Skylark 4 in 15 minutes and derig and have it in the trailer in less than 10 - with 1 other person who is not an Olypic weightlifter. The stories about needing an hour to rig one imply major finger problems.