GOLDE sunroof
 

It seems my P1800E "with original (knackered) Webasto sunroof" actually has a Golde sunroof, which the trimmers are struggling with. Does anyone have any info regarding how these fit? Or any idea who to go to to find out?

When Golde sunroofs are mentioned they usually mean the metal panel type that slides under the rear part of the roof. This is difficult with an 1800 so I guess you mean a vinyl fold-back type and they do indeed usually get called "Webasto" even though there are other makers. If the folding type, have a look on YouTube. Early VW Beetles were often fitted with these so they get the most mentions. Example here. This is a 2 parter.
https://www.youtube.com/watch?v=EggBPLoMVCw

Talk to these people https://www.thetrimmingcentre.co.uk/tudor-webasto