Well, mixed news today. Gutter repair estimated and approved, but I'm not yet sure when the work will go ahead. Meanwhile, isolating the outside power socket has not prevented the RCD from tripping when there is heavy rain (of which there was more today). I've tracked down another switch that controls either the outside light at the back of the house, or power to the garage, or both; turning that off meant I was able to reset the RCD more-or-less straight away. The outside light hasn't worked for years and as it's just beneath the guttering it is also a candidate culprit. Meanwhile I have to keep fidgeting with the thermostat to convince me the heating is operating properly (so far, so good).