It's a drain for the roof, very common on the N15 for the sunroof models to get blocked and start to rot out from poor drainage so may be worth fitting an airline to the end and giving it a few shots to clean any leaves or gunk or debris that may have gotten washed into the hose
Well I sent airline up it and blew anything out. Has a spiders egg in there all webbed up. Could be a sign why it wasn't draining as well. Anyhow I don't have any drainage channels as such that I can see on the roof. I shall look inside the car where the roof folds down into.