Refund of Computer services done

Hello everyone - wondering you could help me with this, I didnt know where to post it as theres so much here.Anyway:

I want a refund of 50 pounds from a local computer shop - I took it in to have some faulty keyboard keys repaired, they said they were working. I took it home, and they wernt - it was still as faulty as before. For all intents and purposes, they practically did nothing, yet I paid for the service.
--How do I get a refund on this?

Thanks for any help..