GHNL is:
G = MG
H = engine size between 1400 and 1999cc
N = roadster
L = left-hand drive
88485 is in the range for a 1600 engine, all bodies.
May of 1959 to March of 1961 was MK1. Since your GHNL is not followed by a "2", it's a MK1, 1600, late 59 to early 61, but:
Who knows how long it sat at the factory, how long the shipment was, how long on the dock, or at the dealer? Could have been registered and licensed later that that March of 1961 date.