Hi Floyd.
Thanks for your thoughts.
Last night I looked into my DAW and found that for some reason all the individual tracks were somehow combined with each other to varying extents, i.e. the vocals had certain amounts of the bass, drums and other instruments on them.
This was obviously muddying the mix. No idea how it happened.
Anyway I took away all the sends and started again. Seems to have resolved the issue. I hope.

Vic