Vehicle damaged after service

i serviced my vehicle at vw wonderboom 13/06/2017 and 3 weeks after that my car started leaking water, now I did not manage to launch a complain immediately because I was out of town since then up to now, I have refilled my water for 5 to 6 times after that.

now that I am in town I launch a complain to VW wonderboom and they book my car in, but when I get there I get told that for them to diagnose my car I need to pay a fee even though I tried to explain that the problem started 3 weeks after the service, yes I agree 4 months its a lot but I was out of town for work, what could have I done?

my problem is why am I getting charged for a diagnose for something that happened after their service? I would not have a problem if they diagnosed it and charge me for a part if there is any part broken.

the service technician was even telling me that I am not an expert so I can not comment technically about my car.