Thanks all for replies - interesting to see the views expressed...
BEagle like me operates from a RAFFCA club though in his case they have CAS as well. He's quite right, though - we monitor the radio but we can't approve access to the ATZ (how would we do that? illegal air to air comms or illegal unlicensed ground to air?). Fair enough, we were aware of her presence and indeed callsign (and anyway I could read the reg with binos from the clubhouse). I'm not interested in busting her - no useful purpose would be served - hence the reminder on here.
It seems to be a bit of Air Law that many people forget after the exam, that's all.
Tim