How to fill a myson boiler?

Hi

I have a Myson Apollo 30b boiler which continually trips the electricity circuit after 5 - 10 mins of use. The flow pipe from the boiler does not get very hot during this time which would indicate a problem with the heat exchanger.

As this is a sealed system introducing chemicals will be very difficult so am therefore thinking of going down the route of complete drain and refill with clean water. I know where the pipe is to drain however I can not locate the fill sections. I have checked by the water tank and there is no water connection joining the water intake to the heating pipes and there are only the gas, feed and return pipes near the boiler. Anyone have any ideas of where this usually hides?

In addition do the apollo boilers have a pressure gauge valve hidden somewhere?

I can not find the instructions for this boiler but been unable to locate any on the internet - anyone got some in PDF format?

Many Thanks for your time