Hi...moved to this house end Feb. Its an older house (Council built 1950s), there is an outside overflow pipe near roof guttering, just above bathroom window. There has been a steady drip from it pretty much since we moved in, more recently though its more of a flow. Not vast amounts but enough to make me think there might be a problem up there. Where do I start trying to solve this one?!
You will have at least one water tank in there. One will be about the size of a bath. One might be about 1ft x 1ft x 18" They should both have close-fitting plastic lids and insulating jackets, but probably haven't. Look at the drowned spiders, mice, mud and dirt in the bottom if there are no lids, and consider what you have been cleaning your teeth with.
Look into them both with your torch. One will be full to the brim. I hope it is the big one.
Measure the length of the ball-cock because you are going to buy a new one. It will probably be brass and probably look something like this. If it has a round plastic float you can re-use it, if not, a new float is not expensive.
Have a look my link and see if they have a sales counter near you. It shows a part 2 valve which you must have. Observe the plastic bridge on the top. The Prestex is a very good one, you can buy an inferior brand if you want. If you buy it in B&Q it will cost you more than this good one.
Have you got some adjustable spanners, and do you like DIY plumbing? If not, are you willing to learn, or are you going to ask around for a recommended local plumber?