I've got a mate that has some Alloys off a Citroen Saxo, set of 5 with delivery mileage tyres. Looking at the www.tyresave.co.uk/fitments.htmlpage the PCD is the same as Fords, the centre bore is slightly larger but the offset is different, around 20mm instead of 38mm.

Anyone know if this is likely to be an advantage or a disadvantage?

If it is a standard locost you are building (not an Mk Indy or Luego Velocity etc.) then it will be the right offset and pcd but you will need to get some centre bore reducers from Elite alloy wheels or somewhere similar.

Adam