The rules state that you can only use one fire action per unit, I read in the revised rules that Sustained Fire means you can fire twice, is this available to any unit or is it an attribute/skill noted on the unit card?

Can someone explain how SF works for me please?

I think it's called "Sustained Attack" in the rules, not "Sustained Fire".

Take a look at page 7 of the Revised Core Rulebook.  It's explained there.

Essentially, though, the same unit can't attack twice during the same activation.  Instead, they can use both of their actions to perform a "sustained attack" which allows them to do a normal attack, then reroll any blanks.

Agreed ^^

I think conceptually it's easiest imagined as the single attack that it is, but with an accuracy bonus for not moving. 

"Sustained attack" sounds like "attack-attack" or "prolonged attack," (which is maybe what they meant), but "Aimed attack" might have been more consistent with the way it is rolled since if you hit on all initial dice, you do not roll more.

Thanks guys, that certainly makes it a lot clearer than it appears to me in the rule book!