First Time Wintering

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A small amount of water in the cassette won't do any harm if it freezes. I live a fair bit north of the A303 and have never had a problem.

I would have thought the water tank would be more of an issue than the cassette. Not leaving too much water in the tank to allow for expansion would be prudent ISTM. Protection of pump diaphragms and other delicate mechanisms may be a different matter.

Paul, I cannot but wonder why you dont leave your cassette empty when not in use, summer or winter!

Just lightly lubricate the seal with olive oil.

I would also recommend that over lay-ups IMO it is best not to close the cassette main flap but leave it just off being closed.

This will help ensure it does not seize up to a level that puts at risk the plastic operating mechanism when you do try to open it, and give the seal a "rest" from part of its compression. 

I leave mine empty and vented as mentioned that way it is soon dry and any biological life put firmly on hold. 

As we live much further north from late October on we empty everything liquid. The cassette is lubricated and left slightly open the water tanks are emptied and if we are not going away for a few weeks we leave the bungs out to air and dry the tanks. We can get a severe frost overnight that has in the past caused damage to the kitchen mixer tap, so taps are also left in the open position once we have drained down. Better to be safe than sorry (and expensive)