To be a bit pedantic, it's not a 'public entertainment licence' which is required. (They're issued by councils for venues hosting plays, live music, etc).
It's a PRS licence which is needed; they're issued directly by the Performing Right Society:
http://www.prsformusic.com/playingbroadcasting online/music_for_businesses/Pages/default.aspx
With specific reference to the tyre fitting depot you mention, a licence would be required even if the music was only audible to employees (and not to customers or passers by). If there were fewer than 5 workers who could hear the music, the current annual fee would be �44 + VAT (if they applied in advance) or �66 + VAT (if they waited until they got found out).
Where there are more employees (or members of the public) who can hear the music, the license fee is calculated based upon the number of people who'll hear the music and the length of time for which they'll be able to listen to it. For example, 16 employees listening to music for 250 days of the year, for just 4 hours each day, requires a licence fee of �851.64 + VAT.
Chris