And here is a processed stack of 59 five-minute exposures, flat-fielded and dark-subtracted. That's almost 5 hours' exposure just for the light frames! At least I think that by now I have my autoguiding pretty well sorted out. The stars remain quite tiny even on a large print but I think I deserve a bit more nebulosity for 5-hours exposure ;-) The light pollution remains very discouraging. I'll have to heave all the equipment to a darker site and try again...

Note added very much later, with the benefit of hindsight. The Astronomik UHC filter lets through OIII and Hb in blue/green but filters out all shorter wavelengths. It also lets through Ha, but that is basically of value only to modifed DSLRs. The blocking of the shorter blue wavelengths by any filter makes it unsuitable for shooting blue reflection nebulae, hence a dumb choice for M45.