Check this one out at Hazebrouck, south of Wormhoudt & Cassel. It looks pretty interesting.
V1 (flying bomb) Vengance weapon site, Hazebrouck
There are stacks of Operation Crossbow files at Kew as well that may be worth checking out.
Cheers - Rob
PS - Found this comment on a site, so could be the one you are looking for.
"The Bois des Huit Rues in Morbecque. (near Hazebrouck): Hidden in a wood, this V1 missile launch pad is one of the best preserved."
On my trip last month we stumbled on the site just by chance. I wanted to photograph the wood as this was where the Germans were massing prior to the attack on Hondeghem defended by 5 Royal Horse Artillery.
It was Andrea (bless her) that said as we drove through the wood, 'Look, there's something in there'.
'I said yeah-They are called trees'
Sadly we didn't have much time spare as I still had Hondeghem to photograph but there is a car park with some info boards etc.
I might have a look in there the next time I'm over.