And then I took it to Auto Lusso, who despite being one of the UK's premier Alfa specialists, couldn't find a misfire (because there were no fault codes) but did offer to do the cambelt on it 'because it's got tippex on the sprockets, so it will have been done wrong' at a cost of only £845.
They did usefully point out that the rear ARB bushes were shagged and need replacing..... only available from Alfa, with the anti-roll bar for £192. I've since found them for a lot less.
Apparently, overall it's in pretty good nick.