It started last week. Damp Thursday morning, noticed a slight "VW camper van" engine noise at 2000rpm, dissappeared as the revs raised.
Then I drove to the Lakes, everything fine.
Last night it was back, it wasn't pulling smoothly.
So today I decided that some new plugs would be a go idea. I was round the Vaux place and I also decided that the cruddy leads should be changed as well. In one of those "try anything" moods I pulled all the leads off and put them back on again. Unfortuntaley one of them stayed stuck to the plug whilst the wire pulled out. So I bodged it back together.
Result - it goes fine now.