Thankfully his gloating only lasted a few minutes and he quickly came up with a perfect solution.
Tom decided rather than do my suggestion of plates and gussets, we'd be better to widen the hole to take a piece of tube the right diameter for the swingarm to move in and a decent width to step out the mount.
We turned the original mount in the lathe to remove any square edges/ pieces of metal, cut a 10cm disc to seat the mount, welded them together and welded that to the tube tacked in the frame (i haven't got any photos of this process)
Tom it all welded together
We removed some of the tube facing the swingarm, allowing the swingarm to come over far enough centering the rear wheel.
And its in!!
I'll be putting a gusset on it to make sure it's strong enough