had this problem on my 04 tdci changed stat, sensor in the head to no avail then found that the return feed on top of the thermastat housing wasnt working

on top of the thermastat housing on the tdci is a small plastic fitting has to pipes coming out of it 1 goes to the head the other goes to the expansion bottle in this fitting there is a small valve remove the fitting and clean can be a little fiddly to get of as it has a silly little circlip holding it on.

after id removed mine and cleaned temp gauge went bk to normal went bk up and sat in the middle at all times

hope this helps some of you!!

Graham