Test as presented

We MOT them first find anything that maybe wrong and then service/repair them. Based on experience that is probably the best way to go about it. For guidance purposes see my other thread "Fiat Punto" and you'll see what can go wrong when none mot people inspect cars. 

DVSA prefer, and it is only a preference, that cars are tested first. This is so that they can gather more accurate data of how cars are being driven/maintained between tests.

The customer usually wants the service done first to ensure a clean pass so there is often a conflict and a commercial decision has to be made about which side you want to fall on as a rule.

Neither is right or wrong.

Personally, I always preferred to service first, for a couple of reasons. firstly, the customer has a better chance of a clean pass, which most are pleased with. Secondly, and this may seem a bit sinister, but I’ve had cars fail needing a good chunk of work, then the customer doesn’t wish to go ahead with the service losing me 2 hours worth of ramp time. You they chose not to go ahead with the MOT I lose far less

I used to have a boss that would fit four tyres and then tell the customer that it needed a grands worth of welding doing (on a car worth £800). Needless to say, he didn't stay in business long.

When a service is carried out, it would rarely include a rolling road check or emissions test. For that reason I would say MOT first. 'We serviced your car, but now the emissions test has shown problems which are going to be expensive to fix is not something which I think will go over well. Only a remote possibility, but when it does happen, and it can, it's not good.

There was a time back in the day that the statement you made dwain would have worked to your advantage, like when we had manual adjustments for CO%, however, nothing springs to mind nowadays that suggests a service on a everyday vehicle today would have any benefit to an emissions related issue, with the exception of an oil and filter change on a diesel maybe?

DVSA prefer MOT first as it gives a more accurate picture of annual vehicle condition. Service first will or should prevent Failures if carried out correctly.

Less Fails brings 4-2-2 into play again