Mate easy to fix.
Buy a new side tank and oring, yes they have an oring under them.
you also need vice grig screwdriver and some gasket goo.
CAREFULLY go around the tank with the srcewdriver and pry up the tags so they are not quiet straight. Do this by leavering of the next tab inline, if you do it of the tank you will spread the whole frame and thats a pain.
Once you have gotten the bent up enough to pull the tank out remove it and oring.
Now is the perfect time to blast the bottom 5 or so cores as they gum up.
Now it is all clean and you are amazed how much crap came out of the radiator, dry out the oring groove a bit.
test fit the new tank to make sure it will go past the tags (if you take the foam out of the guts of the new tank it will suck in and be a real pain to fit)
goo around the oring seat and push it in then goo the underside of the tank and fit it.
with the vice grip go around and bend the tags in in a sort of hooked motion to push them in a bit. Just a few scattered around to hold it first,and NOT TO TIGHT.
Then go around and do the rest.
Now set the vice grips so it grabs the tabs on the otherside of the radiator (just to get the correct size) now go around about 3 time to makesure all are even, if not happy that it is tight enought then maybe a little more.
reassembele and bleed collant system.
Then run car till hot and radiator car releasing pressure and look for leak.
Cost me $55 for the tank and oring in country QLD so you will get it cheaper.
3RD one i have done and no problems so far.
I'm told that you can only do this once or maybe twice as the alloy tags weaken and you are allowed to break a couple but not near each other.
I have not broken any.
if you have a damaged radiator laying around you can use it's tank to replace the leaking one.