Thanks ap' ----
I have now got the fuselage halves glued together and I've cut out the area that the normal wing fits into. This area includes the original wing root fairing but I won't be using any of this so I've cut out most of it.
Top pic shows the score line that the kit has moulded in, this is where the cut should go, second pic shows what I cut out. When I did this the plastic sort of stress-relieved itself and the horizontal cut ended up bulged out in places. I've rectified that by glueing some brass channel along the edges. I roughed up the inside of the plastic and the face of the channel so the superglue has something better to attach to.
Bottom pic shows the groove in the A400 fuselage that the wheel sponson fits into, I was going to copy this but I think I'll just cut this out and glue it to the outside of the A310 fuselage. I'm doing this for a couple of reasons, firstly, the A310 fuselage is 2mm smaller in diameter than the A400 fuselage (76mm as opposed to 78mm) so the sponsons will have to be glued directly to the outside of the A310 fuselage instead of being in a groove. Secondly, the three groups of rectangular holes you see are to locate the undercarriage frames and some bulkheads inside the sponson. Cutting it out will allow me to use the A400 kit features to line up everything as accurate as possible.
The next stage though it to make a half round section of styrene to fill in the cut out wing hole, my first attempt didn't work out very well. First off I've used too thick a sheet (2mm), I thought I read that AiM uses 2mm plastic on all their kits, but measuring the styrene thickness after I cut the hole out, it's only 1mm and then while trying to form the 2mm styrene it wouldn't form very well in one piece so I made a bunch of strips about 50mm wide, then glued these together after I had formed them. Well that was a right cock-up so it's off to 'Plan B'.