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Not that easy to tell from a picture but that valve is likely the mains cold water supply which then runs through a Pressure Relief Valve to the hot water tank and the cold water supply. This mechanism balances the output of the cold supply and hot supply so that thermostatic valves etc. work properly.
It is in practical position for a plumber if he needs to change the Pressure Relief Valve as they can fail shut and reduce the pressure to the whole hot water system but not the central heating loop. It is very unlikely it has any bearing on your radiator pressure.
As others have mentioned you need to find a point where the cold water supply is connected to the central heating loop. The pressure of that loop is the reading you are seeing on your boiler. Occasionally these are in really stupid locations and very rarely the flexible pipe is missing or disconnected (good practice is to disconnect it when not in use but 99% of systems I see are always connected).
It doesn't look like your boiler has an internal filling loop and I can't see anything in your pictures that looks like one either. Google flexible filling loop to see what they look like.
Your system boiler and Megaflo is probably the best solution for the number of bathrooms and occupants but it's the smallest size of system that would need one i.e. they are more useful in larger households where there can be multiple demands for hot water. Unless the space is worth a lot of money I would stick with this system.
Airhead-
Ok - thanks, I appreciate this (and the other) replies, and apologies for taking over this thread slightly, this will be my last question!
Is there any chance the attached photo is the flexible filling hose? The pipe on the left (red) is warm to the touch, the one on the right (blue) is cold. It's behind the mega flo, about 4-5 metres away from the boiler. The red pipe splits into the floor, and into one of the connections on the front of the mega flo, the blue pipe comes from the wall and splits into the jumble of pipes on the right of the mega flo.
We probably will stick with this setup, it's just initially disapointing that it's going to take up the only space in a new kitchen where we can have tall cabinets.
christianSpaceman
Thanks all - really appreciate the help. We're currently getting quotes to replace it with a combi boiler instead.
What's really annoying is that I *think the boiler is a combi boiler - it's got outputs with pictures of radiators and taps, but only one is connected, so it could have been plumbed up more sensibly (or to my mid more sensibly).
I think I might get in touch with the previous owner or arrange a plumber visit.
Unless anyone reckons this is a candidate?! The pipe is cold...