I agree that the basic location of the tunnel was known because the start room was also known so it didn't take a lot of effort to know where to put the digger scoop.
I have a copy of the 1977 BBC programme which is also on YouTube
cheers! i see that chane four made a picture on the camp in the early 2000's, but have not seen it. see that some prof from uni. has written a paper on the camp, in which he gets it all wrong. so no change there!