Thanks for all your replies.
My layout is EM, so will that help a little in preventing buffer lock? ie. less side to side wheel slop than OO.
"What I do is crop near 2mm off the pockets, and ditto from the coupler tails, such that the 'bumper bar' of the Bachmann miniature tension lock is in the same plane as the buffer heads."
That sounds quite interesting, I had thought about using the Bachmann tension lock couplings, but try and get them to couple closer.