The Sunbeam was never a popular classic among old bike enthusiasts. As Dirkie says, chuck in the fact that the audience is dying off and things aren't rosy for old brit tin.
Id suggest around the 4 grand mark, give or take a bit each way to account for foibles on the day, isn't unrealistic. However, don't be surprised if it takes a long time to get an offer and the price has to be dropped to generate interest.
If it were a Late 80's Suzuki GSXR slabbie or RG500 in that sort of condition that'd be 20 grand, but sadly its a less than popular brand from an era of machine that is becoming steadily less sought after. I quite like it and wouldnt say no, but im a weirdo.
Edit - DVLA shows its a 1951 model. It looks honest, but even assuming its a sound bike its going to need 2 tyres, a thorough going over by someone that knows what theyre looking at and a service. Call it a grand if nothing major is found wanting and you paid someone to do the work and you wont be far off.
