I've just bought a used set of Mondeo Zetec alloys to replace the steel wheels it was fitted with, I am led to believe that the alloys use different nuts? Does anyone have any ideas of the quality of aftermarket parts if I require replacements or do I try to get a used set of genuine nuts?

Yes the nuts are indeed different.

I bought a set of steel wheels for winter tyres, I got the steel wheel nuts off a car being broken up for spares so they are genuine.

Thanks for that, just done a bit of digging and realised that it is indeed the case just never gave it a thought when I bought them.

I know a place that breaks Ford's nearby so will pop in and sort it out,

Aftermarket nuts are readily available but are they of the quality of qenuine nuts? I doubt it to be honest.

it's easy enough to make a set of M12 x 1.5 nuts, but it's easy to cut the quality of the material and costs of the treatments to get the tensile strength and finish required.

Answered my own question then really!!!