Hmm... I'm not sure I should admit to this but here goes... NB - This was a long time ago when I was very young.... before uk.d-i-y...!
This is my one and only failure ever with DIY plumbing..
Mixer tap in kitchen was playing up on the hot water side. I reseated, cleaned/greased all the bits to no avail. I came to the conclusion that it was an air lock - there are some horrid flat runs to the kitchen...
So...
I decided to pressurise the hot side using the mains. Impossible at the kitchen tap by virtue of the design of the mixer tap.
So...
I went to the loft and 30mins later the hot water overflow to the header tank had a gate valve on it, and the mains was temporarialy tapped into the hot system behind a stop valve ...
With my wife in the kitchen I gently opened the stop valve... "Coming through yet?" ... "No... Well... Err.. " (typical :-) open the valve a little further... then a little further... hmmm... why ain't this working ?
Then - slight creaking noise from the hot system... decide that's enough and lets turn off the stop valve... too late! Boom! ...... in fact almighty BOOM !!!!!
Stop valve turned off - go to explore....
Turns out that the hot cylinder in the airing cupboard had decided enough was enough... You may have noticed that the bottoms of cylinders are concave... Not any more on this one... Convex. - not only that but the cylinder had moved up by six inches... all the joints to it were now at a slight angle !
Lucily there was not a single leak.... I spent the following day replacing the cylinder... It did actually need replacing !!!
I realise your situation is a little different but BEWARE...
Oh - the tap... yes - fixed by replacing the tap... it was knackered...
Dare I ever post again to this NG.... :-(
Roy