Next I believe I am going to try using a micro controller to better manage the conversion as it appears that the signals have a bit more of a timing requirement rather than being purely logic driven. Think the ATTiny2313 for this task as it doesn't need too many pins and I don't think there will be too much processing power required.
I have also been working on the keyboard controller. This is purely theoretical at the moment as the Spectrum is doing the weird graphics stuff but this appears to be decoding the keyboard signals and mapping them to the correct responses.
I think tomorrow I will be attaching that to my other Spectrum to test it out.