Anyone know good places to ride in Northern France?

Hello all,

I've not posted for a while but have been enjoying reading the new site, anyway I was hoping some kind well informed soul could point me in the direction of a good place to spend the weekend on the MTB in France.

I only want to drive about 4 hours from Calais which I know is quite flat which is why I'm finding it hard to find the right place.

Thanks in advance.