There is a place known as graffiti tunnel just on the A50 near your known dogging spot.
If you want to get a group of people together for photo's in the under ground bit, I can ask my gaffer, he runs the security for that site.
Otherwise, if your after local local stuff, I can see if MFN's grounds are free on the 4th and then could possibly get some photos next to the lock or in other various places.