F22. Standard error code Nails. Serious loss of pressure.
Sounds like it's far more than a leak.
System pressure rises, excess pressure blows the water out of the expansion pipe.
This pipe goes through the wall to outside. The usual thing is to stick a milk bottle over the end, then come back later to see if it's full.
Causes probably... faulty pressure sensor; faulty pressure safety valve; lack of air pressure in the expansion vessel, or a ruptured diaphragm inside the expansion vessel.
Something diabolical must have happened to stop you from re-pressurising.......... unless, the heat exchanger has had a serious hole in it, and the water is going straight down the condensate pipework (inside the boiler itself)