Many thanks for your response guys.
Just to confirm, the films I developed are Kodacolor II which is C41 process.
I'm confident that correct camera exposure was used at the time, so I can only think that they've not aged very well.
If it's possible to get more reliable results developing them as b&w, then I would. I just want to get a reasonable scanned image from them.
I also have 4 C22 process films from the 70's, and don't know what to do with those!