OK, to shake things up a little!
Most 300's will have had a timing belt replacement by now.
Early 300's had a different timing belt assembly which was no good. the bad design caused belts to break prematurley.
the difference is essentialy in the bottom pully and the idler; the early 300 had no shoulders on the bottom drive pully which caused the belt to wander and tear. the later version had shoulders to rectify this.
What does this mean?
your's is an 'M' reg wich means is had the 'OLD' style kit from the factory. If you order your new kit 'mail order' by 'VIN number' there's a good chance the old kit will turn up.
But until you take it apart, you wont know!
I suggest getting the parts for the Idler and tensioner roller localy. and having a usable car available when you do the work in case you need to run out and swap them... as they may appear to be the wrong ones when you go to change them.
The biggest give away as to whether the engine on it's last belt change was given the conversion to the new kit will be the shoulders on the bottom belt pully.
if you are given the option when you order the kit, go for the newer style of tensioner and idler. then pay attention to the torque settings because the new kit was documented with different values.
Do some research and all will become clear!