Thanks for all the comments folks,
I've given it a proper clean and sanded all contact points. Reassembled carefully and it still doesn't work.
Very weird - it's as if the circular dial is a few degrees out when the unit is assembled. I can get the ignition to turn on when using only the plastic part at the bottom, but when I insert this back into the key mechanism (that has an asymmetric male tab) the key won't turn the ignition on.
I guess they do wear out, and that's why those replacements are available!
If it comes to it, I'll order one from the USA, but ideally I could get this fixed asap. Thanks for sharing the links!