Many thanks for the sage advice regarding the motor and indeed perils of trying to nail one on the back of a SR5.4.
Following a great deal of head scratching I went down the bracket route so that the A frame with associated lights n stuff could stay in place......so far so good, however there was a 'foul' with the A frame bracket. Using my not considerable DIY skills I manufactured a plastic spacer plate that allowed me to fit the motor bracket tight to the A frame bracket. (Hope the photo comes thru)
Bolting everything together and then dropping on the motor it actually all hung together quite well, with the spare lifted the main motor can go lock to lock without any clashes and with the spare dropped and the bracket fully deployed I can go lock to lock without the tiller snagging the A frame.
So a qualified success I think and a little peace of mind for the worried partners of the 'Duo-divers, three nutters in a RIB trip' (my good lady refers to it as 'Brokeback Mountain in Neoprene', can't think why) which is planned for round Mull in August!