How would an MOT inspector know
anything about belt tension or condition on a M3W which is a very unique application? Unless you are at very high mileage the belt is likely fine as is. The belts are very durable and long lasting as long as the tension and alignment are both in a reasonable range. The original alloy rear sprockets were a different story and wore fairly quickly and were later replaced with steel. It is possible that a very worn alloy sprocket can do damage to the belt teeth, so look for that if you have or had an alloy rear sprocket fitted.
To answer the replacement question, if you have a jack and a couple of stands you can do a belt R&R yourself (search this forum for details). It does require removing the rear wheel and swing arm, so it's a bit involved, but can be done in a few hours. Belt alignment takes some nuance, but again not too difficult. You might wait and evaluate all things belt related until the rear tire needs replaced, unless the belt is manifesting problems at present.
Also see:
https://www.mtwc.co.uk/wp-content/uploads/2018/01/M3W-Rear-Hub-and-Sprocket.compressed.pdfwhich will give you an indication of the amount and type of work to be done.
