My first thought was to tighten all the bolts and stays but upon inspection all looked well. There is one part of the rear mudguard that rivets rather than blots and that seemed to be the culprit.
Clicking on to the Brilliant Bikes website (if you are in the UK or perhaps beyond, I do think that they really do live up to their name) I ordered a new rear mudguard for £10.
Fitting a rear mudguard is pretty easy but does require the rear wheel and brake callipers coming off. When I first bought my Brompton, this job would have been near impossible for me to do but over the years I have accumulated enough knowledge to tackle most jobs.
I suspect the old mudguard must have had some sort of knock unbeknownst to me as it became very obvious once it was off just how much play there was. I now have a rattle-free mudguard and normal service has resumed.
Stay safe people!!