Bath tap problem - help?

We have a mixer/shower tap on our bath - one of those with a lever that switches between tap and shower.
The tap works perfectly, but the washer that provides the seal to stop the shower working is screwed on to its mount and it has become detached. Net result when any attempt is made to use the shower the water pushes the washer upwards and seals the outlet.
To get at the washer I need to remove the upright which has the shower handset cradle on the top and the hose outlet on the side. No matter what I do I cannot get this to unscrew. The 'fawcet' part - the bath outlet - on the opposite side of the head inscrews without difficulty.
Anyone any ideas how to get the thing apart? I've had it in the vice and (with a piece of wood as a buffer) used a hammer, I've put penetrating fluid on it, but all to no avail. It is now becoming a nuisance not being able to have a shower.
Before anyone says get a new tap, I don't know the make of the original, and it does not rise vertically from the attachment points on the bath - it is slightly inset. Given the problems I have had trying to get replacement tap valves for it I don't hold out much hope of replacing the whole thing.
Er, help?

harrogate three at ntlworld dot com
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