Now some further info which will no doubt show my lack of intelligence :-/
I have now raised the co-driver seat at the front to the top hole which has rotated the whole seat backwards (if you see what I mean).
This replicates a more reclining aspect and has improved head clearance no end.
Measurement from top of seat "headrest" was 97mm to the roof, now it's 130mm :o
It also has the effect of moving the co-driver further back away from the driver's line of sight etc :)