Noggin,
As far as I am aware the PFA coaches have not got an ANO exemption to receive remuneration, which is why they have to call the money they get "expenses".
The point relevent to this thread however is could an instructor give his services to a flying club or group for free, but still claim expenses? For example could the club re-imburse the cost of attending an instructor seminar without it being counted as remuneration (and therefore aerial work)?
The answer to this would be of interest to me.