Good News! The operator was at Prestwick, not Aldegrove, and so we may yet get the answer on that code.
Having now checked the map, Prestwick Airport is about 45 miles from the end of the Mull, so no operator there could have seen an IFF code either. As you can see, it's not about nobody being bothered to check the IFF code, but simply that we will never know what squawk was in use, the code found in the wreckage may or may not be the one that ZD576 was 'squawking' and we have categoric statements that ZD576 was not capable of deriving range from an IFF 'beacon' in the way you describe. Please now let this particular hypothesis rest, as bidden by several posters already.
