Absolutely brilliant, thanks so much.
We took the the old thermostat out last year and it's taken until now to get round to fitting the new one. Normally he's a really meticulous guy and he was sure that the capillary tube was routed up and down through the fins for maximum exposure, but we couldn't see how.
So he was wrong, no problem, it's now fitted as per your photo and is working fine.
He's now scratching his head ... 'What was I thinking?' Bless him.
Thanks again.