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Cheers guys, just to be clear there is never any dew while I'm imaging, but it forms readily if I cool down from ambient to operating temp of say -25C. This repeated condensation/evaporation cycle on every cooldown obviously leaves some residue on the CCD window which was what caused my problem.
The solution is to cool the camera to for example 0C and after a little while, when the electronics have heated the interior, it is safe to set the desired operating temparature - no dew will then form on the window.
I think this is mostly a problem in humid areas where dew is more likely to form.
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