Water Isolation

[Shiney46]

I am in the process of trying to change my bathroom taps. There are inline cutoffs to each tap but one of them is immovable. There is a 'stopcock' in the airing cupboard which I believe will cutoff the water supply and also feeds the combi boiler, but this too is solid.
What can I do? is there a lock off on this stopcock, as there seems to be a 'nut', could this be to lock the water pressure leading in?

Thanks in anticipation

Bob

 

[JPC]

if its a combi just turn the water off at the stopcock for the house

under the sink ??

once closed open all taps to let water run downwards and change the taps