I put one in Jim, wasn't a problem to install, but the over the shoulder part is a pain.
I had to fiddle with the two 'direction' screws which deal with the inertia part of the recoil, for a good hour before I got it right, 50 pence piece did come in handy.
Anyway, I snagged a retainer that bolted to one of the three bolts that the canopy bolts to, and solved the problem of it staying where it needed to.