As Chris say's if it's the gland leaking it is easily fixed .. and you dont even have to turn the water off. If you look closely at the tap to check where the water is leaking from. more often than not, it seaps from the gland.
Have a look at this clip, it shows you how to repack a gland... I know it's not a stop tap, but a radiator valve works on exactly the same principal. But you will see the similarity and it's easily done in less than 5 minutes. The only tools need are an adjustable spanner and a roll of PTFE tape.
Dont worry, you can safely remove the gland nut without water escaping.
When the gland nut is re-tightened dont overdo it, or the tap wont turn.
https://www.youtube.com/watch?v=5k0OUcFxQiU