I'd argue that to detain you for two hours just for a safety check is worthy of a complaint.
It is correct that it is an offence to refuse to stop when indicated to do so for a roadside check by the police ( and VOSA too )
Having stopped you can ask that the officer or person who is to undertake the test produce to you their authority to test and inspect vehicles - no authority, no test.
There's a fair bit more involved in relation to what can and cannot be done under these circumstances but having carried out numerous similar tests myself ( not focussing on one form of transport I hasten to add ) I would say that taking what Shaun was told at face value the Police would find it very difficult to defend detaining a number of vehicles for such a length of time.
you generally run it so that there is one vehicle waiting for each tester whilst they work on the current one....