Ok the electrician/plumber
came today. (I got him in to look at some plumbing and grabbed him for a chat about the electrics, so no testing was done).
He says there are two slots available on the consumer unit.
I mentioned that the yellow box is not fused and he agreed.
I turned off the yellow box switch and the oil burner still fired up, it has just occurred to me that I have no idea where the wires for the burner go... must revisit that.
I then mentioned that I wanted to run a single phase 18amp compressor in the garage.
He says that he could connect the yellow switch to a spare fuse slot in the consumer unit, run the compressor off that, or put a dedicated 20 amp breaker in (I think, I wasn't following him to be honest).
I then mentioned that I would also like some 13 amp sockets in the garage,
he said that in that case I could have a dedicated box fitted in the garage to cover all that - I think on a 30amp thingy next to the main consumer unit. I've forgotten the details because I am electrical illiterate!
Asked about a price but he said he'd have to price up the parts first, once I knew what I wanted.
Clear as mud.