Anyone that can assess an aircrafts height as being 500' purely by eye must have a unique gift!
Could it possibly be that the owner of the house, being wealthy, travels a lot by civvie chopper, and as they more often than not cruise at 500', he decided on that figure when lodging his compensation claim?
Oops! Being cynical again ...............
Exactly, I think the MoD are fairly safe in their defence, even given that the RADALT was duff it will be a simple matter of producing the ATC and barometric input data to the ADR in court, a few sums with regard to rotor downwash and bye bye insurance "claim"....................... oh here are your court costs . I reckon a 500' hover would produce a mere breeze, probably disturb a hair or two. If you are that way inclined a "simplified" way of working it out, but you need some thrust figures