Well bit the bullet last night and cut a hole in the rear bulkhead !
Drilled a very small hole at first and poked a bit of wire thru to judge the clearance and there was loads so I opened it up using the hole cutter to about 20mm.
Still couldn't see much because there is some sound deadening between the panel and fuel tank so I used my smallest pink digit

All clear so out with the tin cutters.
End result was a (fairly) neat hole about 100mm dia and I got the location spot on in front of the shock mount.
After a few minutes of gently turning the nut I realised the bloody thing was just turning the shock so I clamped the shock under the wheel arch with the mole grips and then got the nut off ok.
There was room to put my lh finger on the top of the nut while turning the spanner with the other hand. Couldn't get a socket on it or a ratchet ring 'cos the top of the mount touches the tank.
I took some piccies and will put them up soon.