Some higher-spec cars have larger brake calipers, meaning they can't take smaller wheels - for example BMW, if they've got the 'Msport brake pack' fitted, it's a minimum of 18" wheels. For normal cars, it's 16".

As to buying new wheels, he'll find that genuine Ford wheels are probably horribly expensive. However, ebay (or other such sites) are great for picking up cheap wheels, and even for selling the old wheels. The bonus for him is that big wheels are 'in' and look good, so sell really well. Smaller wheels not so popular, so can be bought cheaply.

As an example, I bought a 320iSE quite a few years ago that someone had then put silly big 18" Msport wheels on, as opposed to the original 16" rims. I bought a set of 16s off ebay, got new Pirelli tyres fitted, then put them on the car. Total cost about £300. The 18s, with about 3mm tread depth, I then sold for £400 !

So buying used wheels and putting high-quality tyres on will probably be the best bet, if feasible.

Here is a calculator for rolling circumference, and the difference. www.wheelcalc.com/

For the change you've suggested, the difference for speedo, etc will be 0.77%, an ideal choice.

Many thanks both, EBAY looks promising at the moment and currently offers a FOCUS set of wheels. Seems there'll be minimum change to the offset etc. and negligible change to the speedo (reading slightly higher). Had to move, he's just bust another tyre on a pothole - there's millions of them down here. Looking into claiming against the council but unless I can prove beyond all reasonable doubt what specific pot hole caused the damage looks like it'll be a non starter.