This hilarity or delay in actually finding the culprit, regardless of how popular the fault is, was a bit of a pig to find. Reason being was simply that using a hose and spray nossle often failed to get any water into the interior. Rain, however never failed. If I was to do another one, clearly I would go here first but I would likely make up some kind of fake rain cloud with pvc pipework like I had at my disposal in BMW back in the 90's. Anyway the pipe was full of dirt. Sometimes water would make its way into the interior, when i was concentrating on the sunroof with a tape and plasticine barrier to keep it off the windscreen. Other times it wouldn't. The think the pipe would bung up with dirt, then unclog and reclog, giving me false negatives. But to compound the issue the passengers door seal leaks as well. It has been hammered back into place at some point and creased the metal liner. This has left a little gap for water to get from one side to the other. I also noticed the door seal had been silicone'd into place by some bandit previous.

I will be replacing the drivers side outlet nipple thing as well. Another classic piece of British Engineering.

Thanks.