A simple question with a simple answer I hope!

I went to turn off the water supply to my house over the weekend via
the main stop cock which i located on the pavement outside my house
(terrace). When I lifted the metal flap, I noticed that the stop cock
was leaking slightly - dripping not spurting.

Seeing as I like to do my best by the environment and that we always
have water shortages, I woudl like this to b fixed, not to mention if
it gets worse ove time my water pressure to my house will drop?

However, who would be responsible for paying for the repair, me or the
water company (Wessex Water)?

Thanks