Help 300tdi coolant leak from engine

Hi, I hope someone can help. Yesterday we drove about 30 miles and when we stopped there was a lot of hissing going on under the bonnet for ages and we couldn't see where from. We managed to get home OK but the oil pressure light was flickering at idle revs. This morning we think we located the source of the problem - the water reservoir was empty and water was dripping from the back of the engine from a small hole in a circular shiny indented plate. It doesn't look like a new hole, but we'd not been losing water before. See pics below.
We're not very mechanically proficient but looking in the Haynes manual there seems to be no pictures of this region of the engine - having cleaned it all up, it does look different, and there's no obvious way to remove it - but whether it is actually a separate piece or not we're not sure.
There are no Land Rover garages in our area of Portugal and both our mechanic-friends are currently in the UK so we'll probably have to order whatever it is from the UK. In the meantime what do you think of using some radiator sealant stuff? Would it fix the problem and for how long?

Head Gasket i think. the shiny thing looks like a core plug (water) ind if oil pressure has gone also you may have a leak or crack on the head itself! did you notice any smoke from the exhaust? does the oil still look like oil or has it turned a creamy colour these are both indications to the same problem.

though i am not that clued up myself i might be wrong but someone will be along soon no doubt

Many thanks Garry, I think you're right about it being a core plug, I've just looked it up on Wikipedia. I hope you are wrong about the rest of it, sounds expensive! No, the oil is not creamy - could the oil light have been flickering because the engine was getting overheated?