Thanks to everyone who replied, very much appreciated.
After further chuck tests, Mike's and Dave's reply and receiving a reply from Bryan at Barnac in Stamford, it is now clear that Emco, for some mad reason, decided to make a taper that was recessed in to the spindle bore. Whether this was to extend the between centers capacity or to force user to be loyal to their not inexpensive accessories, we'll probably never know.
Yeah I've also got the collet chuck, both bolt-on and 14mmx1 on a non-Emco 2mt arbor and they're a pain to set up, because you've got to take the chuck off.
The reason I need to put these chucks in the C5, is to check their accuracy. This in turn was in response to my new Wabeco drilling poorly from the tailstock, with visible wobble, regardless of drill size or type or if starting with a 10mm spotting drill. It's just one more nightmare in a long line with the D4000.
The tailstock drill chuck flexes by 0.25mm(two fingers on the chuck nose) and that's with the camlock lever and quill lever locked and a quality MT2 collet chuck seated well in the barrel. The arbor and bore are spotless, the ways are spotless and the chucks are a top end Albrecht and Jacobs that both show under a thou run-out.
I've clock the tailstock bore, a centre and a chuck with ground pin. One minute it is under a thou, then it's 7 thou low and 3 thou forward, then it's three high and 7 forward etc. Just driving me mad, because this Wabeco seems totally inconsistent in the tailstock alignment. A test bar shows that the tailstock is parallel and aligned to under 0.01mm. But it still won't drill better than a drunk Otter with a Lidl hammer drill.
Oh well, at least the Emco issue is identified, so I need extended arbors for my chucks in the future, at least that's an improvement. I think I'll either put the D4000 on Ebay or get a pro to set it up, just to find out why it is so poor at drilling a straight hole. In the mean time the Compact 5 will drill a great hole, so I'll just have to confine the new D4000 to polishing work, ha.
Thanks again for your help, cheers, Nick.