As far as I know, the headset is a french headset so the diameter and threading is difference (I think). Does this mean I can't replace with a standard 1inch threaded (or indeed 1inch threadless) headset and furthermore, am I restricted to only using forks with french threading on the steering column?
As Dead Rats
rightly says, both headset and steerer must match - either one standard or the other - but either standard will work in a French frame. So if your fork is French, you need a French (metric) headset (unless you use the bodge pigman
described). If you plan to fit a BSC fork, you need a BSC headset.
Do check before deciding how to proceed though. Later French frames and forks often have BSC threads. By the late eighties, French threads were dying out.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.