Older fork meets younger headset....is the age gap too much? (Compatibility question)

I think there are just 2 sizes for fork races for 1" forks and head sets:
26.4mm is most common. I think Campagnolo is always 26.4mm.
27.0mm is a bit odd, mainly for cheaper bikes.

Thanks DC,

It was a cheapish headset so therein may lie the problem. I just assumed all 1" headset would be compatible with the fork. At least I know now, just a shame the bike shop salesperson didn't....