Thanks randers4, I followed your link. It's good to know there is a problem at their end!
But unfortunately though it now says on the systemstatus report that they've fixed it, I still can't see PhotoStream from my Macs. I've checked all my settings again and restarted, but no joy.
I'll leave it a few more hours and see if it comes back before I raise it with Apple.
Thanks. I ended up having to call Apple and they tried the same thing, but we ended up having to disable PhotoStream and iCloud, clear out all the caches, reset the PRAM, restart in safe mode, restart iCloud, restart the Mac twice more, then finally I got PhotoStream back!
Bit messy but at least it has been fixed.
Again, thanks both of you who offered suggestions.