Infact - looking at the previews of the skins on this site , it appears that they all show two 29th October - is this now a common problem or is my computer in a "groundhog day" mode?

it was summertime wintertime this night in Europe..
Isuppose computers are affected with that

(joke, I have no clue, but I have the same, and I noticed it allready for several months)

ah right - that then begs the question as to what happens forthe rest of the year - will the calendar be a day out of sync for the rest of the year?

Only the current/displayed month is affected.

I've fixed this already for 1.8.3. You could download the new calendar plugin from here:
http://evocms.cvs.sourceforge.net/evocms/b2evolution/blogs/plugins/_calendar.plugin.php?view=markup&pathrev=v-1-8

It's work perfectly ! Thanks :)