Mytilus trossulus?

[dave r]

I am no Mytilus expert either, but looking at the photos I would say (judging by the position of the beaks and the straightness or otherwise of the ventral margin) that you have a mixture of both species plus maybe some hybrids.
I know in Loch Awe (?) trossulus lives but only at the rear of the Loch, in the area of lower salinity. Further down and at the mouth edulis takes over but they do hybridise there at the transition zone.
I think they can only be reliably told apart by the soft parts rather like galloprovincialis.
Dave