They need to give a free retest, however there are certain areas/problems that will not count if the car has left the test centre. Nasty places will fail you on something like this to ensure that you stay and pay them to do the work. I failed on a lower ball joint, i checked it when i got home and it was fine apart from a small spli in the gator, which i believe is an advisory, not a fail. I also failed on brake pads, i changed them when i got home, found that the old ones were 3/4 full, i went and had words, only to be told that it was dark behind the wheel and hard to see. Ha, no refund, no retest, just a lot of hate.
My advice is to get a recommendation, regardless of price stick to what you can trust. The good guys will go round the car and show you whats wrong and why.