I've noted from the parts manual there are two types - AC & DC.  I'm reasonably confident I've eliminated the above suggestions and I've just pulled out apart again to double check.  Everything is moving freely by hand.

There are no part numbers on the brackets unfortunately, so I cannot eliminate these as yet.  On RGP there are quite a few posts in relation to this exact issue and one of the suggestions is to replace the plungers (as they also magnetize).  These are cheap and easy (and sourced locally thanks to RTBB!) so I'll give these a go first then take it from there.