I have just had a very nice man fix my boiler* with a bicycle pump.

No really, the expansion vessel had lost pressure which was causing the pressure release valve to empty the heating system into my back garden.
The fix to this is to repressurize the expansion vessel with a bicycle pump.

As a geek, this is somewhat disappointing, as I could have done that.. if I had known.  Ok I didn’t know, and while I did do some research on the web that was one of a few potential solutions so it was best for me to hire someone who has had the required training.

* No he wasn’t from the AA but Centrona Gas Services.. **
** This isn’t much of a plug, more so I remember them for next time..