The urge to tinker is awash. What TT might better mine and why?

I too have the Technics 1200G. Anyone want to buy it at £2,250 + postage? I did intimate that a quality set up LP12 would be much more preferable to your Nottingham. For example, I would certainly not buy a Nottingham over the lp12 as I would need to spend much more  than that to improve on what I have already. I am saving up for a limited edition Avid turntable as I reckon that may be the logical next step up, but it is 4 or 5 times the price of an lp12.
It depends on whether your overall goal is to improve (subjective) or just have a different experience. In my view (and Tom Evans) a good phono is more important than a good cartridge - but just as difficult to choose, well not difficult  for me as I know what I like.

I  have had the GL 99 modified as you can upgrade the idler and bits more than you can the GL75. I certainly think you will not buy a pig in a poke with a good condition GL99 and 479$ in the above advert is a steal if in good nick. I paid £500 for mine but it was refurbished in all super finishes and looks perfect /new

As the owner of a much modified Lenco and four high quality DD turntables, and having owned a Notts Hyperspace before embarking on my odyssey with idler- and direct-drive, I feel confident in saying that any top notch vintage DD or any well developed Lenco or Garrard will certainly sound "different" from what you've got and in my opinion, better.  You keep asking "why", but most of the why is built into your own preferences, so it's hard to say why.  I can only say for myself that the idler and DD options produce music that is more like real.  You can read the cliches about rhythm and pitch elsewhere.

@lewm 

Thank you and I guess I should explain what I mean by "why".
Not asking why will it sound better but why might the technology of a different TT help better the SQ of my present TT.

And yes I fully realise it is all still down to personal preference but with prices of TT running many thousands above mine I am genuinely curious.

You mention dd, something like Denon DP 3000 or similar?

Belts are elastic. Stylus drag is real. ;-)