I'm joking.
I thought I'd check the where all the boiler pipes come up from the boiler into a crawlspace. I noticed that a motorised valve was open that shouldn't have been, it should have been closed. It is on a 22mm flow and return that goes to a heat exchanger at the bottom of the garden. This circuit hasn't been in use for maybe 8 years. I had checked for leaks at the heat exchanger late last night and walked along the pipe run. I knew that because of our water table being so high in the rear garden that any leak of this size would be clearly visible.
Anyway, looking along it today I lifted a section. A JG plastic connector had come apart, presumably in the recent very cold temperatures. Underneath the pipe was a hole that had channelled all the water under the fence into next doors 'garden'.
I think I must have knocked the valve when I put the christmas decorations away!
Thanks again for your help, your much more helpful / less rude than the people on the electrical forum