Thanks for your reply David. I checked the renovation settings in the layout by tracing it back as you described, but it was correct.
What I have since found is that what I was seeing on the screen as a 25% fill was actually just ArchiCAD's adjustment of pen colours (from white to grey). It seems that method 1 that I described will work for vectorial fills, but not solid fills.
But now I am having another problem, where some of my existing elements are working differently to the rest. I have attached two screenshots showing the problem and my renovation filter settings. In both images the ridge tiles and window frames are showing as solid, which is different to everything else (everything in the screenshots is existing). Screenshot 2 shows what I am trying to achieve, with a vectorial fill modified to look solid (although it's a bit hard to tell except for the shadows, but it works on the layout).
But of course these settings create another problem, since renovation override styles cannot be changed between filters, where my existing walls in plan will have their original fills, the same as new elements.
I hope I've made clear what I am trying to achieve, so can anyone suggest a way of doing it which is quicker than manually applying a fill over the elevation?