Hi Richard, Welcome! Others will probably stop by and help, hopefully my memory of these beasts will help, sadly I no longer have my own. So this is one that has cropped up from time to time quite often. It's also a problem I had on my old CGV (The second one I owned). Same thing, it's a warm start problem and it's very repeatable as you describe. My suspicion though is it was mechanical. When hot there is expansion in most components, and in my thinking it's down to poor compression why it won't start. It just turns over and over, it occasionally tries to catch but then dies (If you listen carefully to the engine you can hear it almost catch then immediately fail and back to starter motor just spinning the engine). If I still had mine I'd be starting to look at the cylinder head gasket or even cylinder rings getting worn and loosing compression when hot. Once engine is running it sort of self-heals as it's not as noticeable but when starting that's when you need it most. BUT after all that, sensors also deteriorate when hot so your thinking of swapping sensors is logical. My first thoughts would be the PRV on end of the diesel rail, the crank and speed sensors too, one by one is sensible here. All of those can affect the startup and are also sensors very prone to temperature related failures. On the radio/CD you should be fine fitting. You'll need the code from the old one though. The codes are normally stored in the ECU and 'pair' when they connect up, fitting a different unit I suspect will trigger the need to enter the code, which if you don't have the original manual/code doc for the donor car might not work, but I'd say give it a go. I recall that relay you mention but cannot for the life of me remember it's function. I'll see if I can find out! Cheers, Andy