Ford 1.8 Zetec cambelt. - Miller
I have a 98 Mondeo with the above engine and the manual quotes cambelt replacement at 80k miles or 5 years.

However as I understand it the current Focus uses exactly the same engine but quotes 100k miles or 10 years. Can anyone explain the reason for this 5 year increase?
Ford 1.8 Zetec cambelt. - Galaxy
Probably not the same engine!

Are the valve covers on your engine black or silver in colour.

Black cover = 10 years or 100,000 miles

Silver Cover = 5 years or 80,000 miles

These are the Ford recommendations, but many would recommend changing the cambelt sooner.

I think the cover colour changed during 1998, so you car could have either. The two engines are quite different, hence the cambelt interval difference.