was it the same fault code, was it an intermittent fault, these are things that also need to be taken into consideration. Did it work ok for a while between gas valve fitted and then pcb?
As stated above if the coil in the cylinder was faulty water would go from HW to the CH system not the other way round due to the HW cold feed tank being higher.
Have you checked the pump, the boiler may be coming on for a short time warms up then shuts off because the pump is not working. Also you could have zone valves on the system that are not opening causing the same situation.
Do you have hot water in the cylinder? If so it could be a faulty 3...
You will need to wire the 'wireless' receiver into the wiring centre, normally located in the airing cupboard, then you can put the wireless part of the roomstat in the room you wanted to, (normally hallway)
I would say that either you have put the pump the incorrect way round or you still have an airlock in the pipe work. Bleeding the radiators doesn't solve this, check for any air vents on the pipe work in the airing cupboard etc. It's a case o persisting with it, add a little more pressure than...