Sam,
We are advised to keep away from the wigtip mount because of the long pushrod. It can cause trouble if disconnects at the wingtip end, and jams the turn (into wing servo direction). I know this has happened at least once (thankfully on the ground).
I am happy to accept that the wingtip is the most simple retro fit, but I am looking for simple (for our regulator to approve).
Following Kents suggestion I have placed the servo in the wing via the inspection hatch. There is space, and thankfully I have small hands.
It looks like I would have to cut a hole in the metal plate at the front of the hatch (looks like the spar). I could then also put 4 holes in the spar, and feed 4 bolts back into the aileron belcrank space.The servo could be bolted onto these bolts using castle nuts ( not enough space to rivet). Would making a hole in the spar (5*3 inches) be permissable. The plans show that the spar has holes in it in all other areas apart from this one!!!
However Mels underseat photos look fairly straightforward (no hole in spar). Both method look very do-able.