I think im getting there now with the CG.
I have the yellow conformal tanks. There isnt any room in front ofthem because of the rudder servo, but there is space laterally for a flat pack of sub c size cells. I just tried tonight and with the 6 cell pack on the right and a 4 cell 2400 pack on the left the cg is spot on!
Unfortunately the 4 cell pack is square not flat and although im happy making battery packs up for e-flight im not risking making my own reciever pack for a jet so im off the order a new one tomorrow.

I also spoke to wren today and they said that theyd heard of people using lipolys with success but that Nimh would give a false low batt warning despite the cells being charged. This was 18 months ago however. Ive got a 7 cell he1100 pack so i might give that a try and see if i get the same problems as that would be some weight saving. 270g down to 140g.