Thanks for all the advice on the Metro so far. Here's where I'm at with it :-
Timing: well the tacho bit of the tachostrobe just ain't working so all I have been able to do is check the BTDC angle at what I guess to be about
1500rpm (sort of just-changed-up-into-a-higher-gear kind of revs?) and there's no particular indication that the timing is off. Timing is advancing as you would expect at higher revs. I'd check the camshaft timing too if I knew how but to be honest the timing never seemed that likely a culprit for low MPG with no other adverse effects evident, it was just something easy to check (or would have been easy with a tacho that worked...).Vacuum advance: I think worrying about this was a red herring. At idle there is no vacuum being supplied to the vacuum advance and no timing advance. At increasing revs there is vacuum coming through, it seems a bit weak but there are no vacuum leaks that I can find anywhere in the vacuum system, and plugging up the other branch of the vacuum system (to the hot/cold changeover flap in the air intake) makes no difference. And what's being fed through is definitely causing a bit of retardation at higher revs (you can see a couple of degrees variation in the timing between the vacuum being connected and not connected at moderately high revs), and a not-leaking-but-partially-blocked vacuum seems unlikely, so I can only conclude that the vacuum advance is working as it should.
Carb adjustment: the various idling checks/adjustments in the HBOL don't seem relevant because the idling is fine. And I can't adjust anything else without a tachometer, CO meter, exhaust analyser etc. so this would appear to be a garage job, which may be the way to go with it - can't see a tune-up being that expensive. However there is a Metro of the same year with the same engine (including the mystery black cylinder which I think is a charcoal canister) sitting temptingly in my local scrappy. Looks like an accident job but didn't check. It's quite tempting to whip the carb off that and do a quick carb swap and see what happens - good idea, bloody stupid idea, or somewhere in between?
Now I probably should have mentioned earlier but the temperature gauge is reading a *little* low - about three-eights of the scale rather than halfway. Nothing I would normally have worried about which is why I hadn't mentioned it. But given the low MPG problem I was going to change the thermostat on general principles - but looking in the HBOL it looks to be a fair job of work, inlet manifold needs to come off. Doable, but is it worth it given the fairly slight low reading, i.e. about an eighth of the scale? (and obviously the temperature measurement system probably doesn't exactly conform to NASA levels of accuracy anyway). How likely is this to be the culprit?
A million thank-yous to anyone who can be bothered to read all this and give any help they can.