Right, I've located the tubes and discovered that the driver's side is blocked. I've been poking it with a long piece of mains electrical earth wire which is strong yet flexible and the ideal thickness to go down the tube from the hole in the sunroof aperture but it felt as though it was coming up against a brick wall... I then decided to remove the tube to investigate further. Sun visor removed, headlining peeled back to reveal the drain tube, a good tug from the bottom of the tube and it came off. Immersed in very hot water and repeated flexing/poking and it's now flowing freely.
Next job is to refit the tube and extend it at the bottom so that it exits the sill
underneath, where it always should have done...
This may involve drilling a hole directly under the present location of the end of the tube- common sense dictates the more direct the route the easier it will be to keep clear from blockages. I'm amazed my car has no rust here and no repairs to the sills anywhere...
Andrew.