Thanks Val, another very useful site. I've already used it to find possible alternatives in case of ferry disruption 27th December.
In working through the list, however, I'm sorry to say it does not to quite do exactly what it says on the tin. I found some sites which said- Closed Christmas to January 6th and others for which the 'open all year' applied only to their rental chalets and not to the tourer pitches, so if you are using the website check each 'possible' individually and carefully. In all the cases above the source website stated open all year but it was only when checking the individual websites I found the exceptions.
So here's hoping for the Met people will be slightly wrong and the Low on Friday to be further north than they currently predict.