Thanks for the tip, but unfortunately that didn't help as Hugh turned out to be the most patronising, arrogant and unhelpful chaps you could wish for at a tech help department.
I have been a lurker here for a good while but now how a snag that am hoping someone can help with:
A bit of a technical Viessmann one...
I have a Viessmann w-100 boiler connected via 4 pipe system to a viessmann 210L unvented cylinder. The whole system was installed by an approved...