I stripped the drill down last night and cleaned out some of the metal filings. I tweaked the bearing, fan and shaft positions until it ran a more freely, care was needed so that the spindle lock would still engage. After about 45 minutes of tuning it was running smoothly again. The brushes looked ok but the commutator looked a bit dirty, not quite sure how to clean that as the motor is mostly encased with a couple of air vents.

Seems to have done the trick for now. Sorry no photos as I was not actually planning to do that job yesterday.