This is Lex's Model 18. The frame dates from 1926; the engine is from late-1928. The carb is a later Amal 289.
It has needed some work since the mineral oil was changed for R40 without dismantling and cleaning the engine. Some mineral oil remained behind, this does not mix with R40 and the mess that resulted wrecked a few parts; it needed new main bearings and a new bigend to get it going again.