Seized shut internal stopcock (on imperial pipe supplying outside tap)

[chris1982]

As the weather is no longer so cold I wanted to turn on the outside tap again which I had isolated in November by turning off its internal stopcock.
That stopcock is stuck shut. It sits in the a (former)c fitted with compression fittings on an old copper pipe which I believe is of half-inch imperial size.

Couple of questions:
1. Any tips on freeing it up? (WD40 and gentle tapping I tried already, though will give another go tonight)
2. Could I just replace the insert of the stopcock on the imperial copper pipe with an insert for a 15mm stopcock, or won't that fit?
3. Recently we got a combi boiler conversion done. The fitters soldered just next to the tee supplying this stopcock. Could the heat have caused it to get stuck? (Wish I had checked before they came!)

Many thanks.

 

[Madrab]

If you can isolate the supply to it then just replace it the old 1/2" for a new 15mm stop tap. The old 1/2" and new 15mm pipe sizes are comparable and are interchangeable albeit the new olive might be a bit tight going on. It's the old 3/4" and new 22mm that isn't and needs converted.

 

[chris1982]

Thanks. Access and wiggle room around the stopcock is very limited. There is perhaps only 1cm of pipe either side of the stopcock, one side having the tee (continuing to the combi boiler), the other one a 90 degree elbow and the pipe going through the wall.

So I am not too confident that I will get a new one in without disconnecting the feed to the new boiler, which I really don't want to touch myself...