> been told bits for boxfords will fit straight on apart from backing plate as boxford is whit and southbend is unc.
Correct. Very early Boxfords (1st year IIRC) had a UNC type thread.
> and is it only certain models of boxford that will fit?
Again correct. Model A/B/C/T 4 1/2" with the exception of the nose thread.
> do parts from other lathes fit?
Hercus comes to mind. This is a good place to look http://www.lathes.co.uk/boxford/
Also consider joining the Boxford lathe group on Yahoo, we don't bite, really. OK, the website is poor (OK, an absolute disaster) but use it as a mail list and only use the website for the pretty bits.