welcome, I am also new on here, didn't get any replays to my introduction message though. Anyway I'm not far from Bath, my daughter drives there daily as she studies at the Uni.
My 300tdi diff lock was stuck when I first got my car, but that was because it had never been used, it's a common fault. You can do a search on freeing stuck linkage, there is a video somewhere on the web. As yours has been prepared for the mud, I would have thought that the linkage would be well worked in though?