For the benefit of others of the type of information I was seeking and to 'close the loop' on my own question ...
The valve srainer was not blocked or even dirty beyond a brown coloration of the strainer due to `clayey water` after several water board switch offs due to leaks in the road etc.
The valve fell apart as I unscrewed it ... water had been getting into the spring area where the pressure is set (presumably leaking past the spindle) and the spring had rusted through with that black sludge type corrosion ... loads of it inside.
The valve had not been removed / cleaned in 16 years of usage. Okay I can already hear the proffessionals saying, like ... "ah, but if you had maintained it annually (as is correct) you would have spotted the water leak into the pressure adjustment carrtidge". Yes I agree.
Anyway, there you have it, hope above is useful?
You should source a new valve before you start any dismantling else you may be left without water? I managed to screw the old valve back in. Even without any spring at all and with the spindle / shaft sawn off so the valve would screw in, the carrtidge provided a 'sealing cap'. With the pressure / flow turned right down (and fully off over night and daytime while we were at work) at the main stop cock we managed for 2 nights with a dribble only (strip washes! ha) until the new valve arrived.
I guess the professionals would say above is not reccommedned - I agree - but the valve had failed as fail safe / minimum setting and so the pressure gauge was reading around 0.5 bar. But be careful and don't do what I did unless you know what you're doing and minimise any risk ... e.g don't leave heating / boiler on for hours unattended (unvented tanks can explode like a bomb!)
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