Woodpecker-proof birdbox?

Last year a woodpecker attacked a blue tit's nest in the bird box below.  Click here to see the blog entry of the attacker in action.

This year I have covered the damaged box with old roof tiles, held on with a length of wire.

Not a pretty job, but it remains to be seen whether a blue tit nests here again and if it is safe from woodpecker attack.
Update 23 May 2013.  A pair of bluetits have been flying in and out regularly now for a couple of weeks.  So I guess there are a lot of baby birds in there now.  Success.