Thanks for the ideas guys but….
Telly, Sequence has definitely been rendered at Full
Running on FCP 5.0.4
Prefs have been trashed.
Edit to tape in editing mode, no B&T etc
Going via Decklink SDI to Sony DVW A500P
All Video/audio/output/edit to tape settings are correctly set.
Been through them enough time to make my brain hurt!
What get’s me is that the problem exists on some sequences
and not others even though they were all created in exactly the same way.