It looks like it's either the intermediate gasket or the core plug close to it. You will need to remove the whole timing case to get at it, so quite a bit of work to replace a few pence worth of parts. Best to replace the core plug and gasket at the same time to prevent possible further grief. There are plenty of threads relating to replacing a 200TDi timing belt, which you will need to refer to in order to do this job yourself. There's no real special tools needed to this - just a torque wrench, breaker bar, and a decent set of tools. If the crank pulley is having none of it, then a simple extractor can be made up to remove it.
Les.