Have a look on Partsouq and use your VIN. It should give you the exact part numbers.
If it's just the belt, I'd ask Mercedes or mercedes Newcastle/Edinburgh for a quote via ebay using your reg No. to guarantee the exactly correct part for probably not much money.
I have to ask though, why just the belt?
If the pulleys and tensioners are original (10+ years?) have you considered changing these also?
I've bought a gates kit with belt and all tensioners, and I'm hoping to get those done over this next week, storms, rain and sub-zero temperatures pending. To get access, I need to remove the radiator. I'd need to look up whether this is required to replace just the belt (due to very limited access - have a look down the front of the engine!). If it is a lot of work to get to the point of being able to replace the belt, you could consider replacing all the pulleys.