Low pressure in ALPHA CB50 Combi - not working.

Came back from 5 days at in-laws on Wed 27th to find Alpha Combi not working.
Have traced problem (I think) to low pressure : red & amber LEDs flash alternatively and pressure gauge shows slightly under 0.5bar - needle is just in the red zone.
The manual says Combi cuts out when this state occurs, and pressure needs to be increased.
The 'user' manual is not very clear and the diagrams and text seem to use diffrent terms/words for the same part ( - or am I still confused?!)
Underhanging the boiler cabinet there is a red turret valve on the cold water inlet pipe (which has "5bar" printed on it). When I twist the spring-valve I can hear a hissing noise (water flow?) - however this does not appear to have any impact on the pressure gauge reading. I've tried fully opening the valve, and running it for 10secs but still no diff.
Can anyone point me in the right directin please?
Many, many thanks!! ps It's -1C up here.

thats the pressure relief valve

have a look in the faq at the top

kevplumb said:

thats the pressure relief valve

have a look in the faq at the top

Click to expand...

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Ooops!! - told you I was confused.

Though in my defence - and even with hindsight - the manual is pretty awful, from a user's perspective.

I now understand I need to open the valve(s) on the braided silver, flexi-pipe, until the gauge reads just above 1.0 bar.

Thanks very much for your prompt help ... no doubt saved me £100+ call-out ... and frostbite!