Guess what it turned out to be? A rear main seal - of all things - the driveshafts tend to make finding an oil leak kind of tricky cause the oil tends to get flung anywhere and everywhere!
Other common issue is the distributors o-ring - which is fairly high up there too. Aside from that you've got your camshaft seals - which don't often cause problems but it pays to check some of these things. It's really a case of the usual suspects. In the end I replaced every seal in the engine anyway - and then some