Warwick wrote:

While in Japan on holiday in October, I wandered under some of the micer
looking Acers and picked up seeds.

These have been subjected to the British winter as experienced by a seed
sitting in a car glove compartment since.

That probably won't do it. They need to be wet, cold and frozen in soil to
break down the germination inhibitors.

I'm thinking that it'll be soon time to subject them to the warmer
climes of the greenhouse.

Any suggestions?

Stick some in soil now and put them outside to get some freeze thaw action
and rained on. There is still time to do it naturally - otherwise you may
need to stratify them in the fridge. Perhaps try both methods. Natural is
rather uncontrolled and uncontrollable. Unlike common sycamores exotic acers
are a bit more picky about germinating. You should be able to do it. Don't
let them dry out too much once they start coming up. Small seedling trees
tend to be vulnerable to drought (and to over watering).

Regards,
Martin Brown