Thanks for such a quick response. Hopefully you're right and I have a bit more info for this one for anyone else with the problem...

My problem seems very localised to the flat surfaces of the floor-pan. There's no sign of damp on the vertical parts e.g. up to the doors or the sloping part in the front foot well. They all seem dry. I can only imagine there's either some hidden route for the water to run or it slops about when you brake and stuff.

The yellowish foam they seem use under the carpet seems top-drawer at retaining water and of course, it's February and chilly. So I've been using a wet-vac to suck water out of it but it's still pretty wet. I tried those bed mats for kids but I'm sure towels would do as well. I saw another thread and that gave me the idea to use bungee ropes looped round the passenger handles to hold up the carpet and dry it out.

Finally, I found I needed a T40 and a T45 socket (star?spline) to get the the front and back seats out to lift the carpet. The numbers aren't "linear", a T45 is a little bit bigger than T40. Be aware of electrical plugs under the passenger seat. You need a small socket to undo the bolt that holds the plug in place.