Hi Fraser
The plumbers have been and gone (4 hours). Bang on time, clean & professional which makes a change from a typical tutting plumber.
He installed an expansion vessel (which apparently comes supplied in newer megaflos and similar systems) and a pressure gauge. He also redid some of the pipes and balanced out the cold water feed for upstairs which now means no clunking and no big loss of pressure whenever you flush or run a tap! The downstairs pipework still needs to be redone but can only happen once I rip out my kitchen.
I'd give him a call and see what he can do for you. I've been trying to sort this out for 2 years and finally some progress! His name is Arnold and the number is +447795433494.
Let me know how it goes!
Thanks
David