Ok ta. Just had (or not long ago) the breather pipe assembly replaced - as I shattered the original when changing coil packs with Ska last year to diagnose something else. At the time I noted that it was all gunked up so changed the whole thing (it comes as one) and cleaned out the rocker cover connections for it..
Can't see why this would start after changing the battery thats all, and hence the 'what would that have done' question. Its like it drops off and hovers above cuttting out when I slow down, but when I am sitting there (at the lights for example) watching the rev counter, it recovers to a nice steady tick over.
Its not up and down all over the place, but lets say drops to 3-400 revs as I stop, and recovers to 5-600 (or whatever is normal)