Hi TrikkyD,
Since posting I've been able to resolve a few of the parts I need however I'm still trying to find detents and a thumb screw.
I understand Spyder detents are compatible but can't find a UK supplier that lists them. See below for example
The thumb screw is a tougher find. Its minor but if someone has one laying in a box it would be nice to finish off the marker. It probably looks like the screw circled in white; see below. And its to fit here; see red circle below