Hi Davey that sounds spot on the chime release can only take place on the hour to re scync the chime
when it is in the lock position lets have a few pictures of the cam and lever and sliding lock setups so others can reference it there are sure others out there and there is no tech info from smiths at all now. good to know it is sussed out well done you.
It's about 99.99% not a mainspring issue. It should limp along with a 5x weaker spring- poorly but running. Is the train free with the balance and fork out? Like two clicks of wind, train should run and escape wheel should turn backwards at least a little at the end. With just a few clicks of wind the fork should snap back and forth cleanly. With fork out and balance in, balance should oscillate freely and for quite some seconds with a swift twist of the movement. If these things aren't happening then start checking methodically.
Going back to the barrel- it could be an issue: when you assemble it you should hold the assembled barrel between stout tweezers on the ends of the arbor and check for endshake. Make sure the cap is fully seated. Make sure the holes in the plate aren't too worn (often an issue with seiko). But even if all that was wonky it should 'run', at least poorly, with a couple of turns of wind.
I use mother's mag and aluminum polish.others use brasso..others use toothpaste..I just put a little on a clean rag and polish plastic crystals..which you appear to have.It removes the hazing and your crystal will look like new.