Lol, thanks for the replies.
Having messed about a bit more and taken some voltage measurements from the PSU end it turns out it was the connections.
Apparently you can't just try and put 10A through some crocodile clip connections and hope for the best!
Replaced it with proper plug/sockets, wire and solder connections and worked fine.
I was just trying to recharge my unvented hot water cylinder expansion tank using a car tyre pump. Back up to ~3bar