MOTs are important,and are often the only thing standing between an unsafe vehicle and the road.As MOTs are so important,a tester has a certain amount of legal liability, in ensuring that any safety issues are identified.

The extend of the liability may surprise you, and even “untestable” non MOT issues should be spotted by the tester. Otherwise they are not considered to be doing their job with “reasonable skill and care.” This puts added pressure on the tester, however it ensures that unsafe vehicles are less likely to slip through the net.
There are limits to this, and you shouldn’t expect a tester to spot obscure, non-MOT issues. However, glaringly obvious problems, which could cause safety issues later, should be identified by the tester, who should then pass on the information to the owner.
As well as looking for problems beyond the MOT,a tester should ensure that they look after the vehicle while it is being tester.This liability can be sidestepped by making the customer sign a waiver,however,there is still a responsibility to carry out an MOT safely and effectively.If you offer an MOT and service car servicing  service,then,even if the car servicing branch of your business is contracted out to someone else,that it is done with reasonable skill and care, otherwise you will be liable.
Should problems arise during an MOT or service, it can be difficult to ascertain who is responsible, and whether the tester is liable. Broadly speaking,  the tester is liable if there has been an obvious case of negligence and if they have not done their job with “reasonable skill and care.”

Of course things do go wrong, and liability is not always a given, so ensure before signing a waiver or contract with your garage, you read the small print.