Thanks very much to SnitchSeeker we have a bunch of photos from the Swinley Forest filming taking place this week!  See the photos here in the gallery!  Their source also spoke to someone on set and gained the following information:

Firstly he said the reason for the security is that if a person or even just a torch or camera flash gets in a shot it can ruin a whole take. [He] told me they were having 150 extras on set that night, which explained the many mini-buses we saw going through the forest, and the huge catering marquee at the WB base. He said no main cast were due in that night although I noticed on Matt Lewis’s twitter that he was “rockin in from work at 4am”.

They were filming chase scenes with freerunners – the Snatchers? (he called them Catchers) – running down a slope (i think there’s a picture of the slope) towards a crowd of extras, I heard lots of shouting (“Go, go, go!” that kind of thing) which sounded like they were rehearsing the running.

They were going to be filming this till the early hours of the morning, but it eventually would be edited down to a clip less than 30 seconds long. I assume this could only be something for the Battle, with the many people and the section of Hogwarts nearby.

Finally he said that some main cast would definitely be on set filming for about an hour on Monday night, and they hoped to be all done with filming by Tuesday.