Sluggish Dynostart

The Dynostart on my AV105 is becoming increasingly sluggish. Despite two fairly new and well-charged batteries, it failed this morning to turn the engine fast enough to start. It is a very cold morning and my engine has highish compression, but my memory is that it used to be a bit more vigorous. I have certainly started the car in cold weather before.
What should I check first? The brushes seem to be in good order and sliding in their holders, but I have no idea when the commutator was last cleaned. Could it be a build-up of carbon dust between the commutator segments?
Any advice gratefully received.
Dudley

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