coolant leaks

[mzvolane@ozemail.com.au]

Gday listers!
Thanks to those who replied to my earlier queries about coolant expansion tank
p/n and nitrite free coolant.
The job appear to have gone succesfully, I replaced the expansion tank due to
leak on top hose connector and flushed the cooling system with fresh water.
Initialy I did not notice any leaks. But few days later I noticed some coolant
leaks under the car. On close inspection I noticed there is a slight leak
between heater core and top (plastic) section of the radiator also between core
and bottom plastic section of the radiator.
What have I done wrong? I propably pulled too hard when getting the hoses off?
What would be the best way to fix such a leak?
MarkZ