Help please! Have I got a leak?

[easyt]

if the water tank is dripping from the inlet off the mains to it i t could mean a faulty washer in the fill valve not shutting it off fully

and not neccessarily a leak

If you were able to enter the loft and slightly raise the ball float you would know. If it stops - probably a leak. As I said before I doubt that there is a leak. - Always assume the best and check for the worst if pipes are inaccessible and no apparent leak.