Mode C references flight level.
Last time I flew over Bournemouth I was given the usual "not above 2000" - so went across at 18-1900' on QNH (can't remember what it was). I reviewed the flight on the Bournemouth flight tracker website the next day and was surprised to see my alt being reported at 2200' - so it looked like I was busting airspace.
This begs the question, how does ATC know whether someone is infringing or not via a mode C readout? I could fly along the solent at 1900' QNH, and my alt could be showing 2200' on the radar screen....and hence they could think I am inside their CAS??