At what mileage should the cambelt be replaced on a 2002 1800cc turbo diesel focus?

There's no single mileage for cambelt replacement on a 2002 1800cc Ford Focus TDCi. The recommended interval is time-based, not mileage-based. Ford usually specifies a replacement interval of around every 5 years or 75,000 miles, whichever comes first.

However, always consult your owner's manual. It will have the precise recommended replacement interval for your specific vehicle and engine code. Ignoring this recommendation can lead to catastrophic engine failure. It's far better to replace the cambelt slightly early than to risk a broken belt.