Some excellent news
After the renewed speculation about whether the ignition lead was really the whole problem or not, what could be causing throttle lag, whether the exhaust could still be blocked etc, I can confirm again that it's definitely the ignition lead that was the problem as I have again temporarily fixed it.
I took a few pictures this time to show what has been happening as a picture says a thousand words. I was pretty sure that my repair must have failed but I wasn't quite expecting this:

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A hole has been melted right through the rubber where the hole in the cable is.

The rubber looks like it has been hot all over, enough to soften it but it looks like the insulation broke down at the hole in the cable and that the continuous arcing through an initial small pinhole has melted a larger hole, thus the misfire returned...
Here is a picture of the original hole in the cable with the rubber tape removed:

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That's all that has been causing the engine to run so badly for the last few months...
Here's my 2nd attempt at a more sturdy bodge job until I get the time and weather to fit new spark plug leads:

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Three layers thick this time,

and instead of trying to refit it in the guide (impossible with the extra thickness anyway) and risk tearing a hole in it I've just sat it above the guide with some tape around the three at the top to help keep it in place. An advantage to this is its now twice as far from the camshaft cover so less chance of arcing over to ground, (less dielectric stress on the insulation as well) and the top plastic cover presses down onto it slightly holding it in place. I noticed another small nick near the top end of the cable so I've taped that as well this time.
I don't know how long this bodge will last but in the mean time the performance is back to 100% again.
The changes in symptoms are: (that might help anyone else with a similar problem)
1) With the misfire the engine would vibrate/rock quite noticeably on its axis especially if you brought it up to 1500-2000rpm or so, now its absolutely steady with no visible vibration at any rpm. This is probably the best test for identifying an intermittent misfire IMO.
2) Power is dramatically better at all RPM but particularly at low RPM. With the misfire it was really struggling to pull below 2000rpm, all noise but no power, with the problem fixed it pulls freely and smoothly at low RPM, it has a kind of light and effortless feeling now.
3) As well as being smoother the engine is MUCH quieter under load. It was getting quite noisy over the last couple of weeks especially trying to pull at the low end making me wonder if something else was wrong too but no, as soon as the misfire was fixed its quiet again and power delivery is silky smooth.
4) The intermittent throttle lag is gone now too, the throttle is nice and responsive again. I still believe that the source of the throttle lag was the false lean reading from the oxygen sensor (due to one cylinder worth of unburnt oxygen) upsetting the ECU causing it to retard the timing as well as muck up the overall mixture. Because the misfire would come and go with load variations, so too would the ECU's adjustments to the timing.
So as far as I'm concerned its case closed. As soon as the weather picks up a bit I'll order new leads and fit them.
