Roger Tonkin wrote:
Our beans are still red and green, and at the moment show no sign of
ripening/drying on the plants. Given the current weather and the
forecast of possible ground frost at the weekend, plus the fact we go
away for two weeks next Monday, I think I've got a problem.

Possible ground frost? Where? Blimey.
I know it's been feeling a bit chilly, but I didn't think it was that bad!

Beware, last year, I dried them, then stored them in a supposedly
airtight jar, but within a month they turned mouldy and had to be
chucked!

I suggest you didn't dry them enough before storing. I had similar problems
with my broad beans this year. Kept them on a tray for about 2 weeks,
turning regularly, as soon as they went in a jar they started to have the
occasional moudly one. Had to pour them out, pick out the mouldy ones, then
put back in the jar, then keep jigging it about to get the bottom ones to
the top.

Would drying in a very very cool oven work?

Possibly, as long as you weren't planning on using them for seed next year!