The Lotus Grand Tourers

My 1985 Excel has always had slight microblistering on one panel only (the roof beam on the right hand side, over the door and on the C-pillar). With the colder damper weather, this seems to have become much worse.

It doesn’t look like accident repair, as it only affects this one panel, and there is no evidence of overspray.

What can I do about this? Is a respray enough, or could the problem be in the resin itself? Is this panel structural? Can I easily remove it without needing a jig and without taking the rear screen out?

The panel isn’t structural. Held on by screws and silicone adhesive - so in that respect fairly easy to remove. But, and a fairly big one is that you often have to remove the rear side windows to get to the screws. If you’re lucky you might be able to get to the screws through the narrow gap above the window glass.
To do it properly and to last it will need to be sanded down to gel coat, dried (baked) if any dampness in the panel, then a coat of 2k epoxy primer to seal followed by base coat and clear coat (which is also a 2k product). 2k products are hazardous to your health if breathed in.

This can all be done with the panel in place, but depending on the car’s colour the paint will need to matched and/or blended in to adjacent panels - reds and light metallics being the worst to match.