Ah, OK, I don't usually note whether its ADSB or MLAT...
Saw a relatively new French registered helicopter flying over here several times under low clag last year and reporting an altitude of 1000' - the land rises to 574' ASL locally and I saw it come right over the top of me on one occasion as I crossed that summit - it was very low, 300 - 400' sounds about right. Now my understanding is that F/W aircraft should be no less than 1000' over a built up area and helicopters 900' - can I presume that figure should be AGL and ADSB would be reporting ASL?