Aha!

Looking more carefully at what clip appear and which ones dont, Its the ones from the Sony camera that are blank. The clips from the other sources are fine.

When I play back the movie in the preview window thingy the sony clips appear, its just when I render.

One small oddity: One part of one of the Sony source clips does appear. Its a bit that is slowed right down. Dont know if that helps?

I suppose failing averything else I could convert the Sony clips to another format and add them back in, but that would involve a whole load of faffing, re trimming and what not.

Once again, help most greatfully received.