Leaking Stopcock - expensive?

[LisaMed]

Our stopcock has just started leaking. Is this likely to be expensive? And do I need to be careful about a plumber? Is it a big job?

I know nothing about plumbing

yes you need a plumber. If he is competent and experienced it will be fairly easy. It will be more difficult if there is no stopcock or meter in the pavement outside and if your stopcock is fitted tight on the waterpipe where it emerges from a concrete floor so the upstream pipe is inaccessible.