Lee Nowell brought next idea :
I have a wessex lift boy gb1 and it has started slipping on the screw thread.
I assume the plastic thing that runs along it pulling the door has worn.
Anyone had any experience of fixing these? Also, any idea where I can get
parts like this from?

The mechanicals of many such doors come from a common supplier, but
rebadged. There are several garage door specialist parts suppliers on
Ebay. My door is a roller type and I sourced a new motor from one of
them, identical to my failed one.