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Possibly the water intake filter needs cleaning, and if it's not clean then the boiler path for water to be heated will be getting clogged up. Depending on where this may be happening in the system it can result in either low or high pressure... the pressure monitor is one place, so is the clog before/after the monitor.
Shouldn't be too expensive to fix, at worse may involve replacing some other filters and brass fittings as those will have been eroded a little. But this is an eminently fixable issue.
I used this guy for a similar job https://rokas.london/ he was freaking excellent. I couldn't give a line-item cost for this part of the job as I was having a Nest thermostat installed at the time, and the annual inspection, so he did a deal for all of the labour as a single thing and my invoice will be meaningless for you.
What I will say is that he is conscientious, not a crook, definitely knows his business and did an absolutely stellar job.
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This is the guy I've used, was very good. Not sure if he'll cover as far as NW10 but may do.
http://www.full-flame.co.uk/
@plantasia
Usually this but if you've already filled it up, it's probably something else...
Three things it could be: