I was doing orbits over my parents a couple of years ago (Wirral area), someone in the vacinty contacted the CAA and reported me, quoting rule 5 blah blah blah...twas quite a lengthy email, which I recieved a copy of.
TX didnt give any alt info, but Liverpool airport had sound monitoring equipment in that area and concluded from the readings I was 600' AGL, I was asked if I wanted to respond to the busy body who reported me, I did thanking him politely for reminding me of rule 5 and for pointing out to me that I can fly 100 feet lower next time
I didnt have to send anything off to the CAA though, Liverpool ATC had recieved the complaint first and had all the info to hand. It might be worth contacting the unit you were talking to at the time or nearest unit to where you were operating and seeing if they have similiar equipment.