Model M90E.24S

My heater keeps locking out.
Tonight the pressure bar has gone below 1 to around 0.5 and the boiler is working when it feels like it.

I've checked the manual which talks about opening the valves on the filling loop which I'd do but there isn't one. Everything is soldered together.

What there is:

CH return valve
DCW inlet valve (with blue tap)
Gas inlet valve (with yellow tap)
DHW outlet pipe
CH flow valve (fixed no tap)

I know it's only something simple.

McCreery, December 2010