Allen you could be on the right track! I have not made a proper film/exp test(shame on me). It could be the cause, although the image looks OK this is the first time I've use Pyro so don't really know what they should look like. (I feel stupid even writing this!) I'm out of time for the next few days but will make some tests before I blow money on other more expensive alternatives - tubes.
I WILL post my next trial results. Thank you Ian for putting my mind at rest about FP4+