Hi, my parents have the baxi duo-tec combi boiler, and have been having to re-presurise it every morning?

if you shut the flow and return off and run the boiler in hot water mode for a while. if no loss of pressure is noted then you can eliminate the expantion vessel and prv and you will be loking at a leak somewhere in the system .maybe time to call a heating engineer as this is on the wet side of the system a good plumber will do.