Just thought you'd like to know that I think I've found out what the problem was.............head gasket!!!
I noticed an increase amount of steam from the exhaust over the last weak or so and sure enough the water level had gone down. However, I didn't relate the two things (seems strange with hindsight), because the rubber on the rad cap was a bit perished, so I just changed the rad cap. Water level problem solved for a few days. Then I noticed more water loss, investigated a bit further and found water in the oil!!!!!!! Oh joy.
Why didn't I do a compression test?? Presumably this is why I was geting a misfire type situation as water isn't very combustible! Strange that it took so long to show itself? I think the extra pressurisation of system given by the new rad cap caused the gasket to finally blow. Hopefully this was the actual problem and not just a side effect of the underlying cause. On the plus side at least I've caught it early.
Looks like I'm in for a fun Christmas.