Thanks a lot for the comments - if only I had spoken to you before choosing the boiler! I have finally managed to talk to National Grid again and have one of their experts who is looking into the plans for the house to see what size the service pipe is - we can then work out if we need a bigger...
You seem to the the expert on combi boilers and gas pressure and I have been round the houses and back again with everyone I can find (installer, Vaillant experts and Transco) to no avail. I wander if I may ask you a question. I have a vaillant 937 which has a low gas working pressure problem...
