If you don't know when it was changed - change it!!
Also...IMHO....
Regardless of whether you 300Tdi VIN is within the range of the cambelt upgrade kit being required (to stop the belt riding forward into the front cover) I'd fit one of the upgrade kits.
The upgrade kit has the advantage of coming with a new belt, front cover gasket, updated lower pulley and all new idler and tensioners (and usually works out cheaper than buying them separately).
The part number is STC4096L - I'd go for genuine or OE for piece of mind, but there are cheaper versions (but with cheaper pulleys/belts/gaskets). Depending on which kit - you'd be looking at about £90/£100 for OE or about £60 for a cheaper kit.
Fitting it yourself isn't too bad... but life is made a lot easier (!) with the correct locking pins and pullers.
If you got someone else to fit it.... look at what you are getting for your money. A lot of specialists will do it for £100 - £150 but all they change is the belt and not the idlers (which can lead to annoying squeak) - and they will often just refill with the old coolant (which is ok if recently changed). For some people this ok....
But if I was doing it (on my own car, or a customers) I'd fit (well suggest) a complete new timing belt kit, new inner and outer crank seals and a new cam seal if needed, then refill with new coolant and it's good for another 60 odd thousand miles. The price will be higher, but then you've got piece of mind.
At the same time (as the coolant is drained) I'd also check/change the 'P' Gasket and check/change the head to thermostat housing gasket as these also have a habit of leaking.
Ian
