Hi Chris,my engine is a 2000reg ZETEC 'SilverTop', I'm using NGK- take a look here:
http://www.sparkplugs.co.uk/pages/pricing/ccp/NGK-cars-ccp.asp
The length over the thread, from the 'top' of the chamfered seating (where it's at it's widest) to the end of the plug (at the combustion chamber end) is 25mm.
Hope this is helpful, Gerry.

Gerry