I appreciate your recommendation simondi but the glass is badly scratched and condensation is an issue when in damp conditions so the units need changing for a proper complete job ideally!!
That is Rick and you can chat with him on Rover 75 and ZT owners club here: https://www.the75andztclub.co.uk/forum/forumdisplay.php?f=104
He also opened web site for custom gear knobs here: https://customgearknobs.co.uk/