[antonye]

I think you have to cut the unit apart behind the bezel, as it is sealed?
This is how I've seen them done to replace the inner decal with a new colour one (or to switch from KPH to MPH).

I *think* that I have a knackered tacho in the garage which has decent glass in it that someone can have for postage. I know I've got two knackered tachos in the garage, just can't remember if they're both cracked or not!

Although, it might be worth getting a decent piece of glass made up instead as they're obviously prone to cracking!