Phil, As mentioned earlier, (Trev) if assembled correctly your engine should have a circular metal spacer, 42-7509, between the drive side case and the inner primary cover. Listed as a "Joint Washer". Use two gaskets, one each side of this circular spacer, bedded with your favourite jointing. A cereal packet makes good material for these homespun gaskets and should stop the drips.
The primary case should not need O rings under the bolt heads. The chain case gasket seals the mating faces of the the inner and outer cases, this obviously includes the entire area around the bolt holes. When the cover is pulled down nice and tight, oil is excluded from the bolt holes as the gasket settles and seals. O rings will just get squashed. The exception is on earlier models which have cutaways on the internal face of the primary cover casting for level and drain. These bolt positions each need a fibre washer to seal. Thanks "O".
The inner case should bolt to the frame at the rear mounting without being under any strain, as berger says, washers or a spacer sometimes needed here.
Swarfy.