Swiss Plumbing: how to change this valve

Accomodation, House and Garden
1
I wanted to turn off the water to my lavatory cistern, this little valve had a knob on the left side that one turns to off the water. It is ca. 40 years old, the knob would not turn and when I forced it then it broke off. #@#!!

The water outlet to the flush is on the right.

I assume nut A is fixed and does not turn?

I also assume I just put a spanner on nut B and then unscrew the valve?

Of course, the valve will not turn and now I am afraid to put too much force on it or maybe I misunderstand what these nuts are for?

I have a new valve plus the necessary hemp and grease.

The picture is below, any advice is appreciated.

2
A is most likely a lock nut. Loosen it, then try again.

Tom

3
Thanks Tom, a good idea.

I tried it but my size 22 spanner is too big to turn in the hole it jams against the sides.

I will have to look through my tools and finds something to bodge it or at worst buy a crow foot spanner

4
Its been a few days so maybe you already resolved.

It seems unlikely that A is a fixed part. I think the fixture would be shaped differently if that were the case. I would try a spanner/wrench on A while trying to turn B. With the scuff marks on the B, it looks as if you may have already tried this with a set of what I would call "water pump" pliers. So, yes, it could be well stuck. Did you try soaking the threads with a penetrating oil overnight? If that doesn't work, short heat form a torch on the stuck parts can make a difference. When a torch doesn't work, it usually means cutting and replacing something.

5
Rent a plumber for an hour ?

6
Success!

Like Archimedes said; give me a lever big enough.....