Mick mentioned my 'turns' to a colleague who has also had a similar issue, and he said mine sounded identical! He managed to get a tracing done, by walking into A&E whilst it was happening and they got the wires on in time - his is super-ventricular tachycardia aka SVT. It's harmless unless you have underlying heart disease (which I almost certainly don't) and can be treated or just lived with. I am hoping it will be something that simple and harmless!