Hi and thanks for the reply. I suspect the cold joint is so marginal that the contraction of the board in cold weather makes all the difference. As soon as power is applied to the board and it begins to warm up, the joint is reconnected. Two of the joints were easy to fix, but as you say, the third is so close to another component that even with a fine-tipped soldering iron it is not possible to fix properly. I did read that sweating the joints often didn't work and that resoldering was the only option. From underneath that is not a problem, but doesn't make the connection. From above it is impossible unless you are a robot! I might have one last go at it whilst keeping an eye on Ebay. I did try some UK breakers and they quoted between £40 and £120, but when I supplied them with the part number they all went quiet, so I suspect they were quoting for older units from the old style V40.
Thanks for your reply and it is good to know that this isn't an isolated case. One good thing is that I can solder a lot better now than I previously could!