EVERY single thread about UV filters descends into an argument. To save everyone's bother, have a read of this:
http://www.talkphotography.co.uk/threads/recommend-me-a-uv-filter.465906
And then bear in mind that just about everybody agrees that the 100-400 really hates UV/Protective filters. Just google '100-400 UV filter' and you'll find hundreds of posts complaining about that particular combination.
Thanks for all your replies.
It seems that hood on, no filter, and take care. Except perhaps by the sea. Fit a skylight filter.