My leak is very similar, and while I have slopped a bit of oil about over time, mine must be leaking as the level is going down and there's no visible smoke from the exhaust, although I assume she burns a bit.
The top of the engine has a fair amount of plumbing around it as this pic from eBay I acquired shows:
http://www.ebay.co.uk/itm/Vauxhall-Vectra-Engine-2-2-Z22SE-Petrol-Engine-Supply-Fit-/130416869756
Mine also has the pipe work on the the right of the engine which I assume is from the air-con/climate control. How much of all this needs to be disconnected/moved out of the way in order to remove the rocker cover?
Ta