Widgeon,
you are on the right track, but not quite.
Yes to 1 = civil, the weight and 5 = 2 engines (and 0 =1 engine.)
However, military a/c are designated 6 (now, they used to be 5 as in the 565). Coupled to that, if the weight is in double figures, as the NH-90 is, the weight is added to the 6 to keep the 3 number ident.
Classic example is the 225/725 family; it's a 12 tonne class a/c