Brian Gaff has brought this to us :
Probably either the intermittent sparking or the spring tension or length of
the brush. Normally though if you let them go too far the comutator starts to
get eroded away, so best to swap them. In old vacuums they had a similar
thing, but you don't see it these days, probably cos they are chuck it away
and by a new one merchants.

I checked all of that, not realising what the LED meant - it was all
fine, good brushes and perfect com.. The red LED came on soon after
purchase, comes on as soon as power is applied and remains on like the
green power LED.

My best guess, is that it checks and compares resistance through the
brushes and com. and should light up if its too high. It must be a
false indication, so best ignored.