accumulator tank

Under the water supply regulations you able to fit a booster pump to the incoming mains supply pipe as long as it doesn’t pump more than 12l per minute

What is the pressure (....bar)? Flow (.....l/min)? Call another local plumber for second opinion and ask him check mains pressure at washing machine valve and mesure flow rate at kitchen tap again. Do you have any pressure reducing valve on a mains cold water supply pipe? (Could be faulty) Sealed system on old vented pipework could cause leaks!!! Do pressure test on old pipework. I personaly preffer Unvented System. I have two 24l potable water expansion vessels one for cold water only another on a cold water suply to the unvented hot water cylinder in my flat (3rd floor - 1.5 bar mains pressure). 48l in total works as mains water booster.

get another quote,if incoming main is not providing enough pressure have it tested at a time of high usage ie 6am to 9am ,4pm to 9pm,this will show you what you can expect from water main in terms of pressure and volume,water board only has to supply at .7 of a bar that is their legal obligation,most supply at a higher pressure,you need good pressure and volume for an unvented cylinder to work properly,expansion vessels will not increase your water pressure they are there to facilitate expansion,you can fit a mains booster on your main to increase pressure, or as your plumber said an accumulation tank to increase pressure and volume,to do this and fit an unvented cylinder,test pipework and alter pipework as necessary, i would estimate around £4000 to £5000 would be about right,it would be cheaper for you to fit a whole house twin pump to pump to your extension,as long as the cold to extension is not on the main,if it is you could just pump the hot to extension.

yes, it is good idea what you think.