The above controller has recently developed a fault where it won't stop zooming out when I release the thumb-toggle. I then have to quickly reverse it to get it to stop. This happens irrespective of camera in use (Z1 or Canon)
Anyone come across something like this before?

Not that this is any help, but I have found that the the 521 PROi (which is the one you mount on your existing pan handle) can physically jam on the rubber grip if mounted too near it. This produces the same effect.

More usefully, I down loaded a service manual (Manfrotto_Remote_Controllers_521-522-523-524-service manual.pdf) which might help even though it doesn't cover the latest versions. I could send it to you if you like. Sorry I can't remember where I got it from.

Colin,
That would be very useful if you could download it to me.
Many thanks - hope you're keeping warm up there!

Quick update!
Colin, I've managed to follow the link and download it,
Many thanks,
Robin