Any question answered...

I have an old (10 years old-ish) Carbon frame that has separated around the bottom bracket - the BB is made from some kind of alloy. It looks like whatever was bonding the two together has worn out, and the downtube and seattube are a good inch away from the BB with a smaller alloy piping visible in the gap. Is this fixable? And if it is, it is a terrible idea regardless?

Find a carbon specialist and send them some photos. They'll normally let you know the score for free.

If they can't conclusively tell from photos it might cost a few quid to send the frame to them but from my prior investigations the cost is normally in the order of £25 or so (Obv not including P&P there and back).