On a whim I bought some of the Hornby magnetic couplings last week and I’ve been experimenting with them.
I already fitted them to a rake of the cheap MK1 coaches that came with the My Model Railway Village magazine a few years ago. They’re a little too short for those coaches and need to be glued in part way on the NEM pockets. But one fitted they work fine.
I then tried them on the IRM ferts and they seem fine. The coupling bar that IRM provide with the wagons works fine, but I wanted to be able to break the rake down easily for storage. The 17mm Hornby couplers are the perfect size.