Intermittent WI-FI connection issues
I discovered that bouncing airport (Disable/Enable on Air Port Icon in the top right) resolved the issue temporarily.

Eventually I tried a PRAM reset and that seems to have fixed the problem for good.

Would have loved to understand the root cause though ...

There is a collection of fixes on reddit here, that people with similar issues might like to try: http://www.reddit.com/r/apple/comments/2jwah7/psa_yosemite_wifi_issues_list_of_possible_fixes/