I found a 1960 bike on Classic Lightweights s/n 27338, so mine (27250) looks to be 1959/60.
Seems it's a Hurricane: Zephyr and Cyclone had wrapover seat stays; Monsoon didn't have pump pegs or a lamp bracket. Intriguingly the Hurricane was available (up to 1959) as fixed gear: my wheelset (Racelite) is of the period and the DTS bosses and gear hanger look unused.
A couple of things that don't quite tie-in though: now the wheels are off, I have confirmed that it has Campag dropouts and not Agrati; my pump pegs are on the seat tube and the '58 had them on the down tube and by '60/'61 they were on the top tube.
Other discoveries are a Villiers lockring and GB bars. Could the mudguards be original? The '58 catalogue describes them as being of "celluloid": this is early plastic? Any ideas on the chainset?More pics here.