Hi Ed,
have a look here:-
https://www.vintage-radio.net/forum/...=hmv905&page=5
Look at post 97 to see what is on the inside of these pots.
If the C clip is removed and the shaft is allowed to slip backwards, the track wiper washer (for want of a better name) will fall off, which results in more play in the shaft and scoring of the track if the control shaft is rotated.
The key to putting them back together is to remove the retaining nut at the front of the pot (you have to remove the pot from the chassis to get to this).
I do have some spare morganite pots for spare parts. I might have a spare timebase control for your set from the scrap 709, but it would need some sort of repair I expect as the chassis was very rusty. I'll have to have a look in my spares box.
Cheers
Andy