I have the same problem The valve is accessible if you pull off the front covers and then remove the two small screws from the control unit "Pack" . This will then pivot forward and the valve can be seen . if you remove the bottom plate from below you will then have easy access to decouple the valve unit.
not sure about spares I am stripping mine down this weekend and will confirm if the unit can be salvaged In the meantime i have left the mains tap cracked open a trickle so i can at least keep the pressure up and house frome freezing! Brrr
One word of caution when you come to bleed the unit (The instructions are on the back of the "Pack") the bleed valve is located at the very top right of heater matrix housing. it's a bent pipe with a radiator type bleed screw. Nearby is a pressure switch with an aluminium reset rod be careful there are exposed contacts which i found much to my suprise Normally you would turn the power off, but the instruction are vague and say flip the purge swich and then bleed the boiler , I will in future flip the whole thing off before going anywhere near that bleed screw ! Good luck
Martin Sus., February 2007
