I found the source of the water! I unclipped and removed the rear seats then got the carpet up. It was absolutely sodden and there was a huge puddle. I could have drowned in it.
Anyway I squeezed as much out of the carpet as I could and mopped it all up. Before anything else I decided to test the drain holes in the sunroof. Bingo! The rear drain hole isn't working and with the carpet up I saw all the water gushing into the foot well. So now I know what needs to be fixed.
Next question - I thought I knew where the rear drain holes were but was wrong. Where do the rear drains deposit the water?
Cheers,