Re:
So I've managed to fix one. A spring had come out of a mounting so the ratchet wasn't engaging properly.
The other two are toast. I thought I'd take them apart to see what I could see. Both had the same issue - snapped part that pushes the ratchet ring round which was why there was no engagement. My theory is that if you over shift, this part will be pushed past its limit and simply snap when it is pushed against an immovable part of the housing. Bit of a design flaw imho.
At least I can say I tried, and I've learned how these shifters are put together. I must admit I was impressed that there were tiny bearings inside rather than just bushings.
Never had a set of Shimano ones break though, perhaps they are superior in how they are built and handle clumsy users reaching for a gear they don't have?