Oh dear... 

Driving at 70mph-ish, an almighty bang and the engine died. AA suspected the timing chain had snapped, and have been quoted £1,600 for a replacement, before knowing if there is any engine damage (!).. This seemed a lot to me, but seems the engine and gearbox have to come out to replace the chain, and I'm not a mechanic and we trust our garage sooo.... Am I being daft or should I go elsewhere? Would mean towing so there'd be a cost involved there, of course.

It's a 61 plate, 110000 miles, valuation at Parkers not much over £2k, so have decided it's not repairable. Question is - can we sell it as spares/repairs, and what would we ask? Scrap price as a non-runner we've been offered £297.

Pretty sure the tensioner was checked at the last service (carried out by Mini), though of course that doesn't mean anything!

In all honesty, you can pick up another mini for that price. 
I would look for another and then eBay the old one with a reserve of its scrap value or search for another engine. Depends on how much the labour costs are?