Wotan - what you say is correct, but lets be honest, some are more helpful than others. Some give you 24 hours for a free retest, others 2 weeks. Some will tell you it's failed and that's it, others will off helpful advice as to how you can remedy any failures yourself. Some are just plain rude and miserable!
XRJohn - Snuff mill, (round the back of tescos, Berkley Precinct, Ecclesall Road) have always been very good with me and mine, although it's 7 years since I lived in Sheffield, and I don't know if they are even still there.