Didn't quite go to plan - because the emulsion was proper sticky, some of the places where water seeped through the old paint stuck to the sticky roller, leading to much swearing. I'll wait for the new paint to dry, then attack it with a damp cloth and a scraper to see if I can persuade any other flaky bits to come off, then will retry with a brush. on the affected areas. Reasonably happy with the rest of it though.