Chris - I have found another plumber, this time recommended to me, but he's not available until Thursday this week. I'm meeting him at my property midday on Thursday.
The original plumbers did not advise on any existing faults, infact they haven't even given me any paperwork! Yesterday they were trying to tell me they also changed a boiler 'fan' in December, but today they say they didn't. The story keeps changing. I've requested a breakdown from them 3 times now.
I posted on here originally as I was concerned I'd been ripped off, and wanted some advise. Since then there's been some suggestion that the (overcharged) valve installed has been fitted wrongly, which is worrying! I've continued posting as I thought it would be wise to get a second opinion.