Thanks Evan! I'd better double check, in case my info is incorrect about HB837.! I don't think it's a typo, as I know there were three P51C's taken on strength for 541 Sqn., with the serials as shown in my No1 post, but I'll check against my listing of USAAF numbers/RAF serial allocations. If it turns out to be the same aircraft, then I'll have to change to one of the FX numbers! Bit of a coincidence, the serials being so similar. But I've had this before, so no sweat (I hope!).
I remember reading Cheshire's book, years ago, where he mentioned the use of a Mustang. I'm not sure without checking, but I have a feeling Gibson also tried one out for similar purposes.
Cheers mate - time to do some digging!