If the temperature dropped below the freezing point of water, after the rain, the water that remained on the leaves and branches could have frozen later on.

It was warm and despite rain off and on the last few days my tree is still covered in white flaky looking stuff.

Hm... then it might not be snow... a close up picture would perhaps tell us more about it :)

How do I upload a pic from my PC?

Um... from where you are, you'll need to upload to an image host site lsuch as tinypic.com or photobucket.com or p1cture.me. You click on upload and you'll be redirected to a page with the necessary links. Look for "message board" links with the [IMG] tag and paste it on here.