Our new combi is screwed to the wall and looking cool (because the gas isn't connected yet).
A couple of things I noted from reading the installation manual (which I reckon no self respecting plumber ever does).
(1) If there is a non-return valve anywhere in the cold feed (and one is allegedly often included in a water meter) then there must be a small expansion vessel in the cold feed after the non-return valve and before the boiler to protect against over pressure. I think that water softeners may also include a non-return valve to prevent softened water going back into the main, but I'm not certain.
(2) Softened water must NOT be used in the central heating circuit because it can damage the aluminium heat exchanger. So when filling the system the water softener should be bypassed. I assume a minor re-pressurisation would not cause a problem. I now wonder what the heat exchanger for the hot water is made of.
Cheers
Dave R