Many thanks everybody for thoughts and discussion. A few points from me on some of what's been said.
Other than the correct wording should of course have been "pan pan, pan pan, pan pan", not "Tower, please consider this a pan", which is definitely not my finest ever piece of RT - I think that the main issue here, and with other urgencies / emergencies is that the pilot passed clear information about the situation and any help they needed. I *think* I did that, if a little scruffily. I think it was defensibly either a mayday or a pan, on the whole I tend to agree with BPF and say that I should have called mayday - but above all I needed to explain the situation to those who could support me, and I'm content I did that.
Re: first aid. I can see the point about the first instructor including the student (unconscious) in his class, at the same time it was a group of martial artists who were used to a degree of use of each other to demonstrate "stuff" in ways that wouldn't be normal in most environments. I am personally more critical of the second instructor for not actively controlling the environment beyond their immediate casualty, nor in providing a useful debrief afterwards. [Incidentally, the epipen needle in question had embedded itself in the bone in the fellow's finger! - he was fiddling with it, not appreciating it wasn't a practice model, plenty of lessons in that which don't need discussing! I gather that when he was sent to A&E, they did make their views on that quite clear].
In the meantime, turns out my student was recording the flight in SkyDemon and emailed me this, if anybody's interested Turweston is 438ft amsl with a 1000ft circuit height, so I kept creeping up but never actually made circuit height - I just kept climbing until I could make the runway, then reduced power. Well inside the circuit, prioritising having somewhere to land at all times. I'm pretty comfortable with that aspect. Also to make sense of the numbers - it was an AA5 with about 10 kts westerly wind at the surface, nearer 20kts within the circuit. The right turn after take-off is clearly the point I took control.
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G