The flexible braided hose is for topping up pressure to 1 bar. The black levers in Vertical position is open position. Open slightly till pressure is at 1bar and closed again. If boiler wont reset try switching it off at the mains switch of the wall for a few minutes.
Sorry i apologise. I didnt realise you were the person looking advice. I thougth it was someone having a go. You should be able to remove the plate without touching the gas valve. Have a look. I cant remember ever having to remove a gas valve to remove a plate. Tho cant remember ever cleaning...
Draining a boiler and taking two screws out is hardly major! As for getting charged £170 per hour maybe thats your problem! This site is for advice. If he feels comfortable doing it. Its not rocket science
Open the drain valve below boiler. About a bucket of water should come out then the water will stop. As no air can enter the water wont drain.change your valve then top up water pressure untill 1bar using filling loop
Read the instructions in the manual on how to replace dhw plate heat exchanger. Once you have the the plate removed put in a bucket with boiler water and some citric acid power from your local chemist. Leave i half hour then run it under cold water. Re install and you should be fixed.
If your pump is running its not the pump! More than likely the central heating diaphragm. When the pump runs the diaphragm pushes out make contact with a switch to tell the board that the pump is running
Migth be worth a try as i havent work on this type of boiler but if there like the glowworm which vaillant make theres a filter fitted in the housing beside the rubber hose's which collect sludge. Which will give these symptoms. Also the engineer should be able to enter the installer menu on...