Thanks drzaius and gee-dub. What attracted me was that on the web site it gives some impressive load test results and they have those red marker tools like dowel markers for setting out. Stops sideways movement too.
Gee-dub, I've used a similar one to the metal one you shared but they had 4 screws on each plate and were really difficult to get to line up when working blind. Found Buttonfix on YouTube: https://www.youtube.com/watch?time_continue=3&v=idWObs9FQVI