Hi Ian,
I take it Nick means the 1090 RTLSDR...
Yes Nick, that could be the reason for Chris's problem, or it could be because the units are too close to the transponder which can swamp the receiver front end, or there are several other possibilities - which is why Lee asked for the screenshots to narrow it down.
Still don't know if Chris resolved the problem, or just hasn't got back to the airfield yet. Hopefully he'll read this and let us know.
Regards
Peter