This leaves a gap of about 2mm all around that I'd like to pour a suitable
glue or resin into, drive out the air pockets and hope it'll all hold.

Araldite or similar epoxy won't flow down a gap like this. But if you
arrange the joint so that it is vertical, put a fillet of freshly mixed
araldite over the gap, then heat the tube below with a hot air gun the
araldite will liquify and flow down the gap (rather like end-feed soldering)
before setting. You will need to keep feeding fresh mix to get enough volume
(i.e. penetration). Better to use 24 hour than 5 minute araldite for this.
You will need to do the two "directions" separately. That said, I have never
had much success in repairing buggies: the stresses are high at the joints
and the alloy tubes very prone to fatigue.