Hi Sam,
A few notes for you...
Carb icing suspected but carb heat had no effect.
Make sure you leave it on for a while and don't just pull on and then off fairly quickly.
Suspected choked/choking mags (no leaning option on this one)
The Jabiru engine has dual ignition coils. It does not have traditional magnetos. As the magnets on the flywheel pass the coils, a spark is generated. In theory no battery power is required, however the battery is required (a flat battery is ok) to complete the circuit. When you talk of "choking the mags" you are talking of fouling spark plugs. Indeed this is possible (whilst fairly unlikely) with the Jabiru / bing carby configuration, particularly at high altitude as the bing carby is not altitide compensating. It allegedly is to an extent but practical experience says otherwise. Who knows how your needle and main jet are configured... potentially it's set as rich as hell and you were very rich up top which should show up with rough running. If that's the case, you'll be so rich that you'll be washing your cylinders with fuel and causing excess wear to your rings and cylinders.
so attempted to lean with fuel tap. Fairly difficult – several donkey stops before I found the point on the tap (only 90 degrees between open and closed!) – but again, no real change/improvement.
You could be safely assured that this is not a great idea. What you're doing is running the carby bowl out of fuel and then drip feeding from the tap. If you believe you had a flat battery you'd be in strife if the engine stopped on you!
Problem increasing – so I increased RPM (from 2600 to 3000) and engine ran faster than for a normal cruise but smoothly, EGT etc. all okay.
Then, silence. Long let down to valley floor (doing all the usual things) engine restarted (by itself) at about 500’ agl – I’d have made my field I think…
How did the engine start by itself, was it turning over during the descent? Sure, if it turns over fast enough 280rpm approx, it will indeed start if the ignitions are on.
Closer analysis with a multimeter, battery reading 12.1V and once engine started with external power there was no change to this figure with RPM movement. The aircraft has no electrical system indicators.
30 years flying, and I get two engine failures in one flight!
Sounds like your stator isn't working and you simply aren't getting charge. It's pretty common for the two wires that come out of the stator to touch and short out as the plastic covers on the spade terminals are a very soft clear plastic and they rub and melt through. Either that or you have a wiring issue elsewhere. Check the wires running from the stator to the regulator. The light blue 16 gauge wires are the ones that are connected to the stator.
Consider fitting an AMPS meter or a volts meter as a minimum. There are plenty on EBay, in fact some simply plug into a cigarette lighter socket - can't get much easier than that.
If you are regularly going to fly at high altitudes and you already have EGT gauges, I recommend fitting a Hacman mixture controller. It's a needle valve on the dash with a couple of plastic hoses - one is tapped into the carby vent line and the other to the vacuum under the carby. You will then be able to lean your mixture at higher altitudes which will not only keep the EGT's the same as sea level, but also reduce fuel consumption and premature wear (as above). I have one and it works brilliantly. Very easy to fit and if you buy their version perhaps around $200. You could do it yourself by using hose and a dentist style needle valve with tap - all available from Ebay.