Good evening Mark,
KB is a Liverpool registration. Had the number been KB 596, I might well have guessed 1929. For actual date, see Ian Parsons’ reply below. I cannot find HKB 596 in my old (out of date) lists, which may mean that this is a machine not previously known to the SOC. Machine Registrar Ian Parsons can give you the up to date information, especially if you can supply frame and engine numbers.
The above comments do not detract from the fact that it does look a tidy machine, consistent with c1929 but I have no idea of market prices across the pond.