Thanks for the detailed instructions. I've now successfully adjusted the string heights so I still maintain a decent sustain but without choking strings at any of the upper registers. It's always had superb sustain and I didn't perhaps appreciate how the higher action was encouraging this (perfectly logical, I just hadn't given it a thought), which is why I've been careful in my adjustments. I didn't need to adjust the truss rod.
Now I'd also encountered difficulties with the trem action after changing from Elixir 10s to Newtone 11s with a wound third, which involved some spring tensioning adjustments. I found I didn't like the wound third's lesser volume - never quite understood why a wound third, with its extra metalwork and which sits over the highest of the poles, would have less volume rather than more but I'm told it has something to do with specific string frequencies, rather than amount of magnetised pull on the string - maybe others would understand more about this than me, not being too clever on electrical topics.
Suffice to say I reverted to 10s and after making adjustments for the trem springs trying to get things back as they were, that's when the trem plate problems occurred. I found when pressing on the arm the plate wasn't re-levelling itself correctly - same with raising the arm, so my tuning was settling back slightly flat or sharp accordingly. I fiddled with the two rear bolts but no amount of tweaking seemed to resolve this, which baffled me for a while until today when having another go at sorting it. With the guitar laid flat I realised when operating the arm I could hear a very, very faint creak or squeak. I checked the tension of the arm, wondering if the fibre washers had dried out but then sat back and thought about things.
When I'd initially changed to heavier strings I'd also altered the arm's level to allow for the different tensions as I felt it was angled down too much towards the handle's tip end, so using the Allen key I'd loosened the hex socket holding the arm's base in situ, adjusted the horizontal level and retightened things. I'd not appreciated quite how snug a fit this unit is, sitting into its wooden surround but when making these adjustments, the arm's swivel base had somehow moved outwards towards the woodwork by a hair's breadth and it was now just barely touching the wood. Not enough to make the arm stiffer but enough to affect the plate properly recentreing itself. Simple matter of undoing the nut, pushing the arm's base surround firmly against the metal plate by finger pressure and retightening. This degree of movement wouldn't be measurable without a micrometer but it was enough to resolve things and now the trem works fine.
My apologies for wandering off to another variation on the topic but if anyone else has experienced these problems I thought this latter episode might shed light on the subject.
Thanks again for everyone's help and I'm delighted to report the guitar now plays and sounds fine once more.
Kind regards,
Martyn