Hi Guys,
I "think" I may have found the root of my problem. To cut a long story short I disconnected the vacuum pipe to the fuel pressure regulator, plugged the end and the idle immediately smoothed right out, and the periodic misfire has gone! Idling beautifully, rock steady at 850rpm and nice steady exhaust note!

So then I sucked on the end of the vacuum pipe to the regulator and could taste neat fuel.

What I think is happening is that the regulator diapram has a small hole opening up in it as vacuum increases, which is allowing unmetered fuel into the inlet manifold. Also the vacuum on the diapram is not uniform and the fuel pressure is wavering slightly because of it, and there's also the complication of fuel entering the inlet manifold. Obviously as this fuel is unmetered it's pushing up the emmissions, which the closed loop system can't adjust to.

It's the original regulator, so I'd say after 15 years and 130K miles, it's goosed.

I'm going to fit a new regulator as soon as I can get one, so will report on the results.

Fingers crossed!
Mark.