So I just had a chat with Steve from Chester Automatics about getting spares for the 5HP24 and he definitely knows his subject very well. Apparently these gearboxes are fine running on red ATF (ideally Dexron VI) so that's one less thing to worry about
Since these things are well documented on the web and parts are easily available I'm going to open it up and have a good look at the specific problem areas with the A clutch pack while its on the bench, since I would be properly ****ed if it went bang in six months! Hopefully then I will have a healthy gearbox for at least a few years.