What does this pipe do?!

There is a small 1 inch pipe sticking out of the bottom of the overhang eave on my roof pointing down to the ground. Recently it has been pouring water onto the ground sporadically. The water discharge seems unrelated to rainfall and I haven't been using my central heating or immersion much recently. What is it? I'm in my house about three years and have never noticed it before

Thanks in advance...

overflow from water tank in loft? Get up there and see - You shoudl be able to see where the pipe leads to.

I think that's it and the stopcock is the problemYeah I'll have to borrow a ladder and get up there. There's no light or floor but I'll work it out! Thanks