Thanks! Secondary glazing is essentially just an internal casement window, right? The condensation is usually fine, only if it's totally freezing do I get it a tiny bit.
Thanks. I could see lots of gaps and light around the parting bead, as seen here:
So I sealed the gaps using Schlegel seals. No more light coming through, so I think it's totally sealed.
My solution seems to work, but it's a bit of a "hack". Surely someone must have come up with a design...
what did you do about this in the end? I have the same problem. I can always see a bit of light when I look down at the parting bead where the two sashes meet.