Given the age and mileage on your S-Type, at a minimum the cam covers should be removed so the upper, or secondary timing chain tensioners can be inspected. If they have been replaced, the body of the tensioners will be aluminum. If they have not, they will be a brownish composite material.
At this point I would also advise paying close attention to any engine noise that you might hear immediately after a start. Particularly a cold start, and a start after the engine is hot and parked for about an hour. If any chattering or knocking noise can be heard, but goes away immediately by raising the engine speed to about 1800 RPM and returning to idle, it could indicate weak or failing Variable Valve Timing Units.
In any case, if the tensioners are not aluminum, you should look into having them replaced with the latest design.
Good luck!