Since my original A&E excitement I've now done loads of internet research, spoken to a friendly cardiologist or two, had an echocardiogram (bit like a baby scan but they do your heart and don't tell you if it's a boy or girl) and had a consultation with Leeds-based cardiologist Chris Pepper, who's an expert on arrhythmia and their treatment.
What I've learned:- Atrial flutter is much more common among endurance athletes of a certain age than the population as a whole - four to ten times more common depending on the tests you read about. This seems to be because we end up with "Athlete's Heart" - enlarged overall, and often (due to the combination of endurance and intensive exercise) with "remodelled" atria - and this for some reason (still uncertain) screws up the electrics, and makes us prone to the short-circuiting which causes atrial flutter (the electrical impulses which should spread across the heart just sort of swirl round the left atrium at around 300/min, leading to the150bpm HR which sent me to A&E).
I asked Dr Pepper whether putting my feet up from here on would eliminate the likelihood of recurrence. His response was "dunno", on the basis that they've never managed to persuade a sufficiently large sample group of elderly atrial fluttering athletes to de-train. Commenting that "my job is to enable you to carry on doing what you want to do", he outlined the suggested treatment which is called "catheter ablation" (look it up!). The good news is that this is highly effective (well over 90% sorted on first attempt); the less good news is that it has no impact on atrial fibrillation and other conditions which may still occur as you get older. He recommended treatment after a first occurrence "if you're the sort who wakes in the morning worrying how your heart is" but suggested "wait and see" was otherwise fine if all else was okay.
My current status is that my ECG's and echocardiogram show abnormalities of the sort that are still abnormal in Athlete's Heart - mostly this seems to be thickening of the heart muscle. This could be due to genetic factors or a history of high blood pressure (and would then be worrying) but is more likely to be due to near-maxHR stuff on the turbo three days a week for the past decade. However a further consultation (with the NHS cardiologist) and probably an MRI scan are pending to clarify that. If all's well with that I'll be back to full volume (although with a return to racing postponed until 2017 unless performance recovers unexpectedly quickly), and meantime it's a couple of steady rides a week to maintain sanity and minimise the "Jan Ullrich in winter" effect. Will report further then.
Meantime if any of you want a little light reading:-
http://bjsm.bmj.com/content/46/Suppl_1/i37.fulland http://heart.bmj.com/content/97/11/918.short give you plenty to go at!