Many thank you's for your replies. This has interested me alot as I had been wondering the specifics as to a non-mk3 expert the Hornby 1999 tooling looks fine. (minus the coupling and glazing).
I now fully understand the massive benefits a re-tooled Mk3 would bring to allowing for correct formations, especially between LHCS and HST.
In summary it would seem the Mk3 tooling would benefit from:
Correct roof ventulation
Underframe differences between types - I honestly thought the 1999 tooling looked good, until after reading comments above I looked on Flickr and did see differences.
Lima/Hornby tooling differences to be corrected - TGS chunky windows and RFM Buffet Oddity - I agree here, the non-1999 toolings do look rather dated and at least need to be tooled to the same standard as the 1999 versions.
Interiors - modern seating and stickers (curtains?)
Coupling
Disc Brakes on wheels
Better flushed glazing
I certainly see how a re-tool could be welcome now and not upset modellers who already own a number of Mk3s and which not to replace them all. As the re-tool sounds as it would not greatly change the over all appearance of the coach, but would add subtle detailing. This might be in marked difference to the Mk2D tooling which looks old and crude, I have sold on any of my Mk2Ds in expectation of varstly improved Mk2E/Fs.
I suspect Hornby shall not be doing a re-tool on the Mk3 (based on other blogs and topics), perhaps another manafacturer shall see the market potential (Dapol seem to have a nice range of N-scale Mk3s). Though I suspect Hornby could include a few of the improvements into its current 1999 toolings; notably; close coupling, disc brakes and interiors. I shall see what Hornby have to stay to my email.
Very interesting and thank you for your inputs.