It could be the rocker cover gasket, it's a sh*tty rubber thing that cracks under the heat. There is a special corner in front of the distributor (or where they used to be) where they seep. If you let your engine warm up for about 15 minutes and if the bolts can tighten up anymore, then it might be that, they have to be dead tight as the whole thing is pressurised and it pushes the oil out (and there's a lot of oil flicking about on the camshafts).
If there's any coming from the sump, accept it - most Vauxhalls leak from there a couple of drips a day, even new ones, nobody has yet to fix it.
"C20 LET bang"