Blimey, diagrams and everything: thanks very much to all of you!
Boxing it in seems the way. I now need 600mm clearance at the front of the boiler for servicing, according to the manual. And the bog's in the way ... but that's another story.
I've just had my Worcester-Bosch combi boiler repaired by an engineer from the company, fixing a leak left by one of their previous engineers over the weekend. It's fixed, but he said that the first bloke should never have touched it in the first place because of the proximity to the basin under...