My Onan manual shows the following service places in the UK, both on the other side of Offa's Dyke:
Cummins Power Generation Ltd. tel. 01843-255000
Dudleys American Motorhomes, tel. 01993-703774
My owners manuals aren't much help in identifying parts, so we'll have to hope that someone with the same generator can look at that diode and read off the part number.
Another option might be a board from a scrapped generator, if you can find one at a scrap yard. But that might be tough over there.
Yet another thought - try emailing one of the part coordinators at one of the U.S. Cummins locations. e.g. my nearest one is
Cummins West; Click on the Contacts link to see the email links. They'll need model and serial numbers.
Ned makes a good point about the possibility of something else being the cause of the diode failure, although I've seen folks have similar problems which were not caused by something else. Some years ago a friend built a replacement control board from scratch by literally copying the board in my generator on a prior coach. Popped it in and it worked just fine from then on.