Adrian,
From memory BSA never made an adjustable buttplate for the MK 2. A Mk 4/5 plate would fit, I'd expect screw spacing to be the same, but wouldn't bet my life on it. Other simple options are the Anschutz 4709, and "Bisley" brand. Either may require plugging one of the holes for the original plate and re-drilling. These are both limited in vertical movement - if you have a very high right shoulder don't expect full contact with the buttplate.