Mixer Tap Leak 1999 Symbol

My van has a gold coloured mixer tap in kitchen with separate hot & colds tap heads either side of a single housing - water from hot tap dribbles from mixer spout constantly when pump is left on - when pump turned off flow slows to drip and eventually stops after about 45 minutes - I assumed cause is a worn washer, and having replaced washers in domestic taps several times I tried to do so with the van tap - but ..... Despite using a variety of spanners  I cannot get the valve housing to unscrew - my first thought was that it has seized through age - but have since wondered if for some reason van taps are not intended to be taken apart (perhaps due to presence of micro switch??) - I wonder if anyone can advise me please .

N.B. Note to webmaster - I posted a similarly worded enquiry yesterday, and had one reply, but cannot now see it on the forum - apologies if I have slipped up here, but I am new to this.

Responding to my own post - I have now discovered that I somehow managed to get my original topic yesterday on to the 'Monocoque' section of the forum by mistake, so at least that explains that one!   

As regards the main issue of the dribbling tap, the vehicle handbook appears to suggest it has a 'pressure sensitive' pump system, which from research on the www would seem to rule out the presence of micro switch within the tap, so I'm still none the wiser as to why I can't get the valve unscrewed let alone stop the  dribble - any suggestions gratefully received

Not sure of the layout of your tap, BrianB, but I have successfully un-seized tap valves by pouring a kettle of boiling water over the housing first ..  .. expands the housing enough to allow you to unscrew the internals.

good luck

Many thanks for the tip Bob - van out today for MOT and base vehicle service, but I will give that a try when I get it back