Well Ed, I found the opposite !

In a sample of 5 cheap TRVs from B and Q, costing about £5 each, 3 failed
within a couple of months - I took them back with the receipt to get them
replaced and they had stopped selling them - "too many returns" the resident
plumber bloke said....

So I had to get some more expensive ones at about £9 each - problem was they
were a different size and the plumbing was well bedded in to the wall, so I had
to dig out the wall to modify, then make good the wall. The replacements are
still going well after two years.

I vowed to myself that I would only buy the better ones (not cheapest) from now
on.

Nick