They had them on the stand at Beaulieu and although when I asked if there was a possibility of making the 'non-R' suffix type of body and jet block for Nortons, they said "probably not", there was no suggestion that the range was going to be discontinued.
I spent some time at Burlen Fuels recently. They are still making a range of 276 carbs. Although they have no plans yet for 274 or 275 or the long neck carbs as fitted to Enfields and SV Triumphs. But they are in the process of making an adaptor to fit a standard 276 for the long neck application.
Any parts that Surrey Cycles sell will probably come from Burlen anyway.