When should timing belt be changed on a rover 75 2L V6 PETROL 2001?

Re: Rover 75, 2.0v6 petrol engine. The manufacturer states the belt(s) (there are 3 of them) should be changed at either 6 years or 90,000 mile interval (whichever is sooner). Quote for job from ex Rover dealership in Northumberland is £473 complete (inclusive of parts, 7 hours abour and vat).