I tried. And tried. And failed...
...To get the airbag light on my previous Primera sorted. So I have done a bit of research on the subject and can offer an opinion.
Usually an airbag light that suddenly starts to change behaviour will be down to a loose connection. Those connections that get moved and come loose are below the seats. Find them and clean them with switch cleaner (or WD40). Hopefully this will solve the problem.
Regarding the MOT, since 6-4-12 an airbag light must illuminate for the diagnostic stage (normally abut 7 seconds) and then go out. Failure to illuminate, failure to go out or illumination leading to a flashing sequence are now reasons to fail an MOT.
I don't know about the VW ECUs but the Primera was pretty good about forgetting a previous fault. If it did latch one, there was a sequence of switching the ignition on and off that reset the memory and cleared it.
I hope you don't see it again. If you do, I hope you have better luck than I did. (I had to sell mine in the end).