Well, we have now cancelled our visit by a local VHF expert as I discovered loads of water in the deck connector for the VHF aerial at the foot of the mast. Dried it out and re-made the connection and the VHF is sound as a pound.
BUT . . . still total interference when the LED tricolour is on . . . so I have been up the mast and replaced it with the old bulb.
I would like to say it has all been a learning experience, but really it's all just been a PITA. Getting used to shinning up the mast ladder though - the whole operation took maybe five minutes, and I remembered to check the lights were working while I was up there.