Seems the bracket for the extra idler has moved out of line; turns out that one of the studs into the engine block has sheared (b*gger!) and will need removing before I'm mobile again.
So, having accumulated necessary tools and gadgets (thingy to drill at 90 degrees, eeziouts, 10mm HT setscrews....) I set about it today.
Sheared stud was in THE most awkward position between the block and the rear bulkhead, so drilling a hole centrally in it with a thingy to allow you to drill at 90 degrees really wasn't that easy. Anyway, perseverance paid off. Next came the eeziout......not the most appropriate name IMHO. Again, due to the limited access (and not helped by the hole being not quite vertical in the stud) I couldn't get a tommy bar on it. That meant a mole wrench to grip the top, and painfully slow extraction a quarter turn at a time.
Good new is.....all safely extracted, bracket now secured with new HT setscews, drive belt back on (and content to stay on when engine is running, which is a huge bonus ).
Now got to clean it (and of course a quick test run.....would be rude not to!!)