I also purchased a replacement Km dial from lockwood and had to figure out how to fit it as it had no instructions how to disassemble the speedo.
It is a VDO Viewline gauge and is almost 100% plastic including the bezel. The viewline has snap on interchangeable bezels that can be removed by prying off from the rear with a thin blade using the groove to work around the circumference. Once this is off the double skinned lens is laser welded to the body, but i found that a thin craft knife blade carefully worked around and gently twisted between the lens and body broke the weld in 10mm increments, then you are able to remove it, The needle removal I found best using 2 teaspoons.. tips placed under the needle at right angles to the pointer and lever it off.
The face is removed by rotating it and pulling it away. Put on the new one and push on the needle at the zero mark, It is a stepper motor so you cant go wrong, At this stage I temporarily taped the lens to the body and refitted it and rode it to check all was ok. Then used silicone rtv to re seal the lens to the body then refitted the bezel and good as new.
Hope this helps any one of you that is still wanting to try it.
I also have the info for recalibration from VDO if anybody needs it.