I said it would be a long wait... and with the awful weather we've had so far this year it was nearly six months before I made any progress.
Checking this again, it's the rear clamshell which is going to need most paintwork. While I had finished the floor piece of the clamshell, the main part was just in white primer. Finally we got some sprayable weather and I'm pleased to say that I got the inside of the clamshell painted in Halfords Flat Black (which is pretty shiny for a supposedly matt finish). Masking-wise this was the easy step as I only had to put masking tape over the outside. It didn't even matter if some paint came through the various vents as the outside will be painted black anyway.
Probably of far more interest though was that this week I attacked the decals in earnest, and the front half of the body is now fully decalled up. I said earlier that I was struggling to match the decals with the kit instructions and they seemed different somehow - I was definitely doing Indycals a disservice with that comment as it turns out the reason for the decals not matching is that the Tamiya kit is for the '84 car, but I had an attack of numptiness and ordered the decals for the '85 car. Fortunately that's not a big problem, I'll just make the '85 car instead. The only problem seems to be that I've painted the front white stripe a bit narrow - it looks ok but if you put it next to a picture of the real thing you'll be able to tell it's not right.
And that's all I have to show for nearly half a year since the last update. Thanks for looking.