High, as an ex Leigh inhabitant, I thought I ought to suggest:
1) Buy a suitable optical thermometer - Maplin have a good range, mine was £10.00
2) Buy a small tupperware type container with a lid,
3) Expand your test so you have a leakage test, (ensure you include the door release mechanism) and by checking the temperature increase in a fixed volume of water over a set time of operation include a power usage check ( i.e. indicates will cook properly). Buy specific microwave testing labels and always quote the service as a specific test that you will carry out on all microwaves. Charge £5 - £10 per microwave.
4) If you get a leakage failure, give the door seal a good wipe then retest.