I didn't mention it in the first post because it didn't seem relevant, but I've had 2 engineers out - the first showed me how to bring the pressure up by turning the valve on the filling loop, the second disconnected the filling loop and told me I didn't need it but didn't tell me to reconnect...
I have a `Chaffoteaux & Maury Britony Combi 80` combination boiler. Not sure how old it is (at least 5 years old as it came with the house and I have been here 4 and a half years). It has had a lot of problems with losing pressure over the years so I have called a heating engineer in on a number...