Thanks everybody for the feedback, which I've found very useful. I think my view remains that the correct approach is to file a CHIRP rather than airprox report. This is partly because having discussed it with the pilot of the other aircraft at the time, notwithstanding that I wasn't hugely impressed by his attitude towards the whole thing, not having expressed an intention to do so, it would arguably be seen as a somewhat vindictive action. Also, CHIRP reports get read by many more people anyway.
I've once before filed an Airprox (strangely, I was flying the same type, as was the other pilot, although little else was common to the circumstances) which was about 5 years ago whilst flying in Scotland. I was impressed by the thoroughness of the process, but don't feel it's the way ahead here.
A few further thoughts of my own. I don't believe that I and the aircraft owner were at-all confrontational in our discussions with the PA28 pilot, but it's hard for anybody - whether in the right or wrong - not to become defensive when somebody comes and wants to discuss your flying judgement, which is inevitably what we did. The fact that we were both clearly a lot younger than him probably didn't help in his mind at-least.
Regarding liaison with the other pilot on RT, I don't personally think that would have been appropriate given that the AFISO was already trying to tell him that somebody was in front of him and that he should orbit; in my opinion another voice would have added to the other pilots workload and probably increased rather than decreased the risk of something going wrong.
Finally my own view on mixed types in the circuit. I fly from Popham quite regularly (which isn't where the incident happened) where we routinely mix from retract-twins and Yaks, to single seat microlights, without incident. This works so long as the faster aircraft fly a wider circuit and the slower aircraft a tighter circuit, and nobody overtakes on circuit position, so that we're all taking about the same time around - it works at Popham and at Boscombe Down we used to share Air-Cadet mtorogliders and fast jets by a similar method, just requires good spacial awareness and airmanship. I did fly for a while at an airfield that had a separate (parallel) microlight landing strip, and it was a blasted nuisance. It meant that we couldn't fly proper circuits or touch-and-goes, some types you weren't sure which bit of grass they were lined up on because they were calling "XX-final" regardless; all things considered a nice idea that just didn't work in practice.
G
N.B. I wasn't scared, probably because I was too busy being annoyed.