When you say sleeve joint, is that the downpipe tube that slots inside the exhaust?
If so i can remember having some grief on an exhaust i made 13 years ago, always blowing at turbo joint or downpipe to exhaust plus rear box joint. Tubes undersize too much for the bore, really need to grolly the thing up compressing the outer tube. Will be a game getting off and still may keep on blowing. Maybe two clamps first one clamping a slot in outer tube.
Have you got a flexi on downpipe?
Many producers will not use them this means there will be stress somewhere usually at manifold and or elsewhere. Mines sheared a stud in the manifold to head and using flexi, so have a look.