Coolant leak

[ssd]

three days ago i topped the coolant and perhaps put a little too much in.
drove it to work and came back to loads of coolant underneath the engine bay and the coolant level low warning bong. topped the level and drove home slowly.

started to check for the source of leak found no major signs but still had got a slow coolant leak - check all obvious area from top no sign.

had to remove splash pan and looked underneath...found drain plug head was missing. (looks like it had sheared off in, fact it was sitting on the pan). the drain plug threaded shaft is still in the radiator and it is visible that the coolant is leaking from this point.

how could i fix this? how would i remove the drain plug knowing it has no head? would the head blow off like this because of another prob (like me adding too much coolant)

and is this pointing to a new rad otherwise?

help appreciated.