heeey, success. mirror cleaning etc did the job. the furry balls surrounding the cores of bright stars have pretty much gone - all that is left now is the diffraction from the mirror clips (next job). image shows before and after with stacks of 10 subs from both - under similar sky conditions and with similar linear remapping to show dim bits. I think that the improvement is consistent with the very light film of gunk being the issue - have to keep an eye on optics hygiene from now on, but at least it looks like the coatings are still fine.
thanks for the advice encouragement - and particular thanks Ken for your generous offer.
regards Ray