If you take it to a good garage you wont need to explain much other then the fact its leaking coolant. However, it could be a number of things.
Replace the flange, you will eventually have to do this at some point.
You need these bits:
037 121 132 E - flange
357 121 140 - plug
+ N 903 168 02 - seal
032 121 142 - retaining clip
+ 037 121 687 - seal
N 902 809 04 - bolts
Use a bit of fine wet and dry to clean the mating face of the block up as well (Mechanic should do this)
Also check the other flange on the side of the block that goes to the heater hoses, they leak as well.
Speak to Bb regarding the parts.