Your mechanic's educated guess could be correct, but not necessarily so. As I noted this could simply be a leaking freeze plug. I've had both and they are identical in appearance, that is, both drip down behind the timing belt cover and cannot be seen at the source. One costs $1, the other costs $179 ($184 with gasket sealant) from Parts 4 Diesels (don't know about European sourcing).
You won't know until you pull the plastic timing belt housing off (after removing timing belt, drive and cam shaft pulleys, fuel pump pulley, idlers, etc.) Do not remove the water pump until you spot the leak so you will know if it is the pump or the freeze plug behind the pump.
If you've got an oil leak too, it's easy and cheap to replace the front seals while you've got the timing cover off. (Well, the seals are cheap - as is my labor - don't know about a mechanic's charge if you're not doing the work yourself.)
Good luck with it,