As an update, I have ordered Tempest URHM38S and will chuck out the Champions at the next service. The Champions are obviously all defective, with resistances several times bigger than they should be. They still work (have just flown several hours at FL200) but clearly are on their way out.
Tempest did have some manufacturing problems with these but they are now solved. I got a good detailed response from the chief exec there which is very reassuring.
Best place I found was Saywells in Worthing.