What is the maximum number of pucture repairs an inner tube can take? Any thoughts - my spare tube that I carry in my bag currently has a dozen or so patches on it, but still works fine. I'm tempted to keep going to see how many I can get on there until it get impossible to mend.

What is the most number of patches you reckon a tube can take ?

(And I know new inner tubes are only £5 or so, but a puncture repair costs about 10p if you buy the patches and glue in bulk, so "It's cheaper to buy a new one" is not necessarily true...)
I would carry at least one unpatched, pristine tube as a spare.  Put the patched ones on the bike or keep for repairs at home.  The idea being that when you need it -- in the middle of nowhere, in the dark and pouring rain -- you're guaranteed that the old patches aren't peeling off or weakened and the tube should "just work".  Then swap it for a patched tube when you get home if you like.