As an Vehicle Technician and MOT tester in a previous life, we always used the LEV provided at all times during MOT tests and any other time a car engine had to be run for more than 30 seconds or so, petrol (cat and non-cat) included. How many test bays do you actually have? and of what type? are they traditional bays, or ATL bays? A general rule we followed is that we only ever had 2 tests being carried out at the same time per test bay, with one tester using the ramp for the ramp based checks, and another carrying out the pre-checks, interior checks and emissions. if you don't have LEV, then emissions tests have to be carried out with the vehicle outside the workshop, or at least the exhaust a few metres outside of the workshop. Have you ever seen a diesel engine suffer a runaway situation and run on oil that's gathered in the intercooler pipework? you don't want your staff breathing those fumes in i can tell you!