So i've got a pretty heavy coolant leak on a Defender TD5 03. Around two drips per second.

It looks to me like its coming from a small circle on the engine block, just behind the alternator.

Does anyone have any idea whether:

A) That magic crack filling fluid you add to your coolant will stop it.

B) What it is that the circle cutout is.

C) Where the leak is coming from if it's not that.

Any help is greatly appreciated.

Thanks in advance.

Find a pic on the web and post it up so we can see the area you are talking about.
The rad seals work fine K-seal is very good, but first you need to assess what is actually failing.