Upon your advice and also my instinct I have declined to call that plumber back.
Instead I called another with great reviews to ask if he could fit ne a PRV. Unfortunately he doesn't service my area but he did understand the problem. He used to be an engineer and a hydrodynamics lecturer!
He agreed. Air from a dead leg causing hydrostatic shock due to being on a shared main. Some one else on the shared main flushes the toilet with a dodgy refill valve, I get the rushing sound and a bang, as the trapped air causes a shock wave.
He scoffed at the idea of letting air out of the arrestor.
He recommended testing water pressure at night (that's when the sound is) with a pressure gauge off ebay connected to the wash machine service port to see if the overnight pressure is too high, before fitting a PRV. Id have to do this myself.
He also explained about standing pressure and working pressure.
He agreed that the other plumber was mistaken as you said. He was thinking of pressure
relief valve for a hot water system rather than a reduction /
regulator valve for a cold water main. That's what the new guy said.
This is the most promising local guy I've talked to. We spoke the same language. But he said in all his years he'd never fitted a PRV for that problem.
My toilet flush also makes a noise now and I'm wondering if that might be a contributor. He suggested that it could be or it could be anyone else on the shared main too.
Here's my plan.
1. Get the fill valve on the toilet replaced. I had the washer done and the groaning stopped but I think the whole thing needs replacing. See if that helps as a simple fix.
2. Call the utilities and find the house numbers on the shared main then post a polite letter through their door explaining my problem and seeing if they have any issues too. Maybe if their issues are fixed it might help.
3. Persuade the new guy to source and fit a PRV. He says he only works in my area in the summer when he has less work on. I highly doubt I'll find another plumber round here as clever and copasetic. If
he's never done this before none of the others will be any use.
5. If I can't do that, then digging up the floor boards to find any redundant spur is the last option. Can't afford that but cheaper than asking the water company to put in a new exclusive main.
Phew this is just the gift that keeps on giving. Why is it never easy? It's not like I have any other things to deal with like my deteriorating health or feeling suicidal or anything.