Having looked at the Gibson conversion sheet it seems the Hornby D16 has 3mm axles? If so does this mean Markits 7′ drivers can be used? I note in their old catalogue they had a wheel pack for the D16. I suspect it may have used their LMS 2P wheels.
I assume there is no option for using the conversion axles because the splasher clearance becomes too tight?
Yes, the Hornby D16/3 does have 3mm axles (unlike their J15, which has 2mm). I have converted a couple to EM using the Gibson instructions and wheels and they work well. My only concern with Markits wheels is that I think they are a little thicker than Gibson’s and there may be an issue with the wheels touching the splashers.
I did experiment once with Markits wheels, to convert a Bachmann Ivatt 2MT to EM (I was nervous about converting an outside cylinder Walschaerts valve gear loco using Gibson return cranks) but it was impossible to fit everything behind the slide bars. I had a second attempt with Gibson wheels and (plucking up my courage) return cranks and success! I find that if you use something like a GW wheel press with quartering, Gibson wheels work perfectly well and now I generally don’t use anything else.