When should I have the timing belt changed on my 2005 diesel Vauxhall Astra Estate?

When should I have the timing belt changed on my 2005 diesel Vauxhall Astra Estate? I've had varying advice - four years/40,000 miles, 80,000 miles or 10 years/100,000 miles.
Obviously four years / 40,000 miles plays safer than the others but I don't get a lot of feedback of premature failures. If there are any plastic or nylon jockey pulleys, then failure of these, throwing the belt off, is more likely than failure of the belt itself.
Answered by Honest John on

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