I'm interested. You chaps seem to be arguing about who's at 400 feet, who's at 500 feet, who's in the right, who's in the wrong. And there's suggestion that the CAA would get all excited if a PFL got to 400 feet..
So what instrument are you using to gauge this height? Because I've not seen a rad alt in a spamcan and an altimeter is useless because you don't know the datum. And if you are conducting PFLs with any reference to the altimeter at all you're an idiot! Because you should be looking out of the window judging the circuit. And if you are looking out of the window judging the circuit you will probably see the model. Or the power line. Or whatever other obstruction or hazard is waiting to kill you.
If an aero modeller reports you to the CAA for low flying during a PFL you are a profesional or flying like one and the last thing on your mind at 500 feet over an unknown datum is looking at an altimeter! And you say that to them, and they say "absolutely right sir" and that's the end of that. Maybe don't do PFLs where you know there are aero modellers.