Hi Andy
Unless someone has modified your lathe – forget metric dimensions – they are made to good old imperial. The spindle thread where the chuck fits should be 3/4 inch BSW (10 tpi). The MCHA 2783 is the serial number and a check or enquiry on the "Drummond Lathes" Yahoo group should give you a year of manufacture. You'll also find a lot of information about these machines and the other Drummond lathes on Tony Griffiths website – http://www.lathes.co.uk.
As Eric (Lampton) above says if you've got the backplate then you can fit a chuck to that – the backplate may need some machining to accept the new chuck, but you can do that on your lathe as the backplate should ideally be machined on the spindle that it's going to be used with.
You'll find that the round beds are a bit idiosyncratic in that the cross slide and top slide are effectively merged into one – that can lead to a bit of head scratching when you come to work out machining operations and sequences, but they are still capable machines. They also have an advantage when you come to mount them in that it is virtually impossible to induce a twist into the bed, unlike most girder type construction lathes. I've mounted mine with the bolt in the tail end mounting casting, that nips on to the lathe bed, deliberately loose so that there isn't any twist.
Good luck with your machine, I hope you get a lot of pleasure from using it.
Keith