aaaaagggghhhhh this happens to my D8R and my Princton tec all the time. Going to try the approach with putting a cold battery in and see if that works in future.
What I normally do and if you break your torch dont come crying to me
is this.
I wrap around the body of the torch a tea towel. I then kneel on the handle and with both hands turn the head of the torch. The tea towel stops your knees hurting. It does take brute strength but works. To stop the head slipping I use some big thick elastic bands. This helps with the grip.
The torch handle is rather strong, at first I thought it would snap but touch wood has not yet done so. It gives that extra bit of leverage if thats the correct term.
When this first happened we had (buddy and me) such a laugh trying to get it undone it took ages. Stupid things. Next time Im going to make sure I get one with pins on the back so it dont have to be taken apart. Well thats if its still tight after the cold battery technic. :o)
Good luck
Jamie