Over the last few months, I have been topping up the pressure on one of the tyres but only every 3 or 4 weeks. The others have kept their pressure.
Yesterday it suddenly lost all pressure whilst driving (not too far from home).
The guys at the tyre centre (a well trusted independent firm) said the rim was corroded and that was causing a gradual leak. They tried their best to get rid of the corrosion but it has lost pressure again really quickly.
Will I need a new wheel?
I had had this problem with older alloys before. When new they will have something like a clear coat paint finish which protects against atmospheric corrosion. This has now gone and tyre fitters will brush goop on the inside rim before fitting a new tyre to compensate but a full wheel refurb might be a better bet - always assuming you don't have a faulty valve or cracked wheel.