Thought you might like an update on my quest for a better sound on the Simon Neil Strat via a new trem unit. We'll after much angst and procrastination, I plumped for a Wilkinson 'ST-Model' unit - technically a T-WVCS-C. This has the heavy, full-sized steel inertia block and US string-spacing, so isn't a direct drop-in replacement, but it made sense for me as I now have both guitars with the same spacing, so swapping between them is much more straightforward.
Fitting the new unit: this involved drilling the existing mounting-screw holes to 6mm, then gluing in 6mm oak dowels. I then simply drilled new screw-holes holes in the new spacing and fitted the bridge - a 1hr job, with no problems (as long as you use some masking tape on the drill bit as a depth-gauge to prevent drilling too deep).
Okay, result-time: immediately fuller, more balanced sound, both acoustically and through the amp. The top strings are now more 'balanced' in tone. The unit also uses a drop-in trem arm, which has now cured any slop - once you've adjusted the tiny Allen-headed grub-screw adjuster which takes up the slack.
So, success at last (and I think the P/Us are in the clear after all)... and a highly recommended mod for all those of you with the cast 'import-spec' blocks.