3 screws visible on the edge once the door is open, then the seat squab needs to come out so the 2 screws holding the backrest can be removed so that can come out, followed by a couple (?) of screws holding the interior trim behind which are IIRC 2 more screws holding the outer panel. The interior trim needs to be pulled forward slightly to get it out.
Thanks guys, got the panel off ok. all screws undid. However the black vinyl covering was very brittle being 47 years old, so soaked the rusty corner in boiling water and was able to slowly peel back past the affected bit, sanded rust off, primed and covered in adhesive and used very carefully a hot air gun (hairdryer would do) to re-soften and fit the vinyl back and 'G' clamped a flat piece of wood over it for a while to get as flat as possible. Quite happy with result all went back without too much swearing. Heated rear screen has never worked, so was able to check supply ok, it was so either the elements or the earth which is presumably on the other side - next job