I had an experience a few years ago when a well known tyre fitting company, after they put a new tyre on, told me my front discs and pads needed changing and they could do it for £X. I was a bit suspicious so I left it. When the annual service came around a few weeks later, my garage found no fault with the brakes. That was 3 years ago and all is still well.
I am also suspicious of "MOT for £30" places. Sounds like another loss leader to me.