What is the point of a site like this if it leads to a bill of nearly £400 without a fix to a problem. Had there been more support I might have been sorted now with less expense.
Try to remember this is a DIY section for non-gas appliance related plumbing & heating. For example, if you wanted advice on changing a rad, planning a bathroom, fixing taps, ballvalves, controling heating etc etc etc. Correctly identifying boiler faults is not feasible without being on-site or communicating with another engineer.
As a non-DIYer with a gas boiler related problem, no-one is obliged to 'support you' or help you knock other tradesmen's prices down.
Your statement implies you hold members of this forum responsible for your £400 bill from a tradesmen you found and none of them know. I suggest your expectations, in this instance, are misguided.