Andrew,
Saracens Head IS a south Lincolnshire group, but they don't seem to have a website.
Maybe the Southern or Northern Federations might have contact details, if you ask
I am sure that if you start asking around Lincolnshire Model Engineering clubs, someone will have a contact..
Maybe someone on here will be able to put you in contact?
They exhibited at the now defunct Spalding Hobbies and Model Engineering Show., and occasionally have met someone who I recognise as a member, although do not know his name.
Mounting a 3 or 4 jaw chuck will require an adaptor which fits the Mandrel bore to carry the chuck.
Once you know what the bore is, a suitable adaptor can be made up (MT stub mandrels are available
(Arc EURO don't sell above 3 MT, but RDG offer a 4 MT. )
Being a Raglan, the mandrel bore may be 4 MT, as "read across" from the centre lathes..
Any info on the Lathes UK site?
Once a backplate is mounted on such an arbor, all machining should ensure that the register is square to the lathe axis, and concentric.
Capstan tooling will be automatically centred of the holder is dowelled to the Capstan head in some way and then bored / reamed from the mandrel..
HTH
Howard