Take off the lid. With some you have to unscrew the silver ring surrounding the push- button. The flap valve is in the middle. the ball-valve to one side. In the middle of the flap valve is a hole. Does the water overflow that? If so change the ball valve, or try to adjust it so the water level...
If you know anyone with a 9" angle grinder, you could cut the CI pipe near to the wall. You can get an extension piece, cut to size then then pan connector suggested by Kev.
Lots of good advice, but none of you warned Nick the dangers. I used to do a hell of a lot of lead work and I would have been very wary of a job like that. If things had gone wrong the neighbours would have lynched him. Firstly if the solder had cracked on the stop-tap, or a drawing fault...
Ring round, I'm pretty sure Plumb Center amongst others, sell copper tails, take the tap with you. If you put a conex fitting on the end you can tighten it enough so that you can put a spanner on the nut, you'll only need a nip.
Copper pushfits are a pig to demount. Buy a Pipeslice, very easy to use in tight corners, cut just above fitting then use a JG Speedfit (much easier to demount if anything goes wrong. Then use any other fittings to get it all back together.
I've never had to bend a spanner to 90, but I had a tricky job where I had to bend one to 45 and it worked great. Hand tight I wouldn't risk, but they only need a nip. Overall tho, I'd advise cut your losses, you've been shafted and buy a tap that's properly designed. BES and Screwfix are...
The original tails are probably shot now. You can get new ones. As a suggestion to tighten them- you know those basin wrenches plumbers use? Get a cheap open-ended spanner, get it redhot just below the jaws, then bend thru 90 degrees. you should be able to turn it using an adjustable on the...
The pressure keeps rising then blows? Could it be the valve to pressurise the boiler is passing? Get the pressure to 1 bar, then turn off the mains water. If the pressure stays steady, that could be the cause.