Pprune Radar
It has been established that ZD576 was not allocated a squawk on departure from Aldergrove and that Aldergrove had no SSR display on its radar.
Also, for topographical reasons, it seems most unlikely that Prestwick Radar might have had radar contact or been able to receive the IFF response. It's likely that the BoI will have already asked for and seen the Prestwick, Aldergrove, Machrahanish and AWACS tapes. If an unusual squawk had been seen on reviewing these tapes, doubtless mention would have been made of it in the original BoI report (remember, the Chairman of the BoI refused to accede to the unreasonable request of their Airships to change his conclusion, so it is most unlikely that he would have agreed to edit out any information about an IFF code!).
I daresay NATS would be prepared to review the appropriate tape again, but this really isn't necessary.