Thanks again - I had a feeling that the freehub body might be interchangeable and vaguely remember that a 10mm allen key was involved, but good to have it confirmed, especially since I have the same prob to sort on my road bike (see Ken Bird thread in the road section).
Still got to find what the hub internals are like of course.
I'm not fussed about keeping to Biopace - the outer ring is virtually unworn so I can keep that and use it to hide round middle and inner rings.
Useful to know the BCD. (I edited this cos I'd put PCD automatically. I think my dad used to call it that - Pitch circle diameter I think, but same thing anyway) I usually take things like that into the shop and ask have you got one of these...
"Any day above the ground is a good day"