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ChasnotRobert
You need to add more water to the heating (radiator) loop to increase the loop pressure. There should be a top up valve somewhere. Quite where will depend on where the plumber installed it.
The “monolith” is the pressurised water tank, the boiler heats this in the same way it heats the radiators. There will also be a small electric immersion heater in there (probably what the two wires are going to under the grey cover).
Don’t mess with the small white pressure vessel, this is an expansion vessel, it doesn’t do anything with regard pressure in the radiator loop.