That (Downton ??) mirror can be bolted through your window catch if you drill the screw hole out slightly. If it's the same as the one on offer over here at Minisp*rt etc the stud will go through the window hole fine.
I mounted mine exactly like Pete has said, by drilling out the window catches to 8mm and shaving one side of the retaining nut so as not to foul the catch. Can take a picture tomorrow for you.
Even after modifying the mirrors so they can be positioned in further however, they still don't offer great visibility so I'm now considering some mounting brackets at the moment like the Swiftune ones available.
I mounted mine exactly like Pete has said, by drilling out the window catches to 8mm and shaving one side of the retaining nut so as not to foul the catch. Can take a picture tomorrow for you.
Even after modifying the mirrors so they can be positioned in further however, they still don't offer great visibility so I'm now considering some mounting brackets at the moment like the Swiftune ones available.