Just tried this now and the E27 and E14 work perfectly so far. Slider range reflects the bulb (2200-4000k) and reacts and responds fine without the previous issues.
Oddly enough I forgot to remove my custom device handler so it applied my tweaked DH by default when adding this again, although without changing my custom DH the slider worked as I expected it to without issue!
Have deleted the custom one now and re-added, picking up ‘ZigBee White Color Temperature Bulb’
Just for my information on this and reading the posts above, this was all done in meta-data behind the scenes specified by the vid? The post linked to by @GSzabados Vid: What's available and what does it do? seems to suggest this wasn’t anything I could have tweaked, but could do in future as things progress?
Appreciate the the support on this one.