So what's the cast time on this? Item says use your action to cast, but the Conjure Elements spell has a 1 minute cast time and says 'as if you had cast the spell'. Which do you use?

The cast time is 1 action. The "as if you had cast" part means look up that spell for the other specifics.

Do i need to concentrate on it?

No the magic item just works but you have to rp and light it on that turn its just the action it's not casting a spell 

I have a question whilst you don’t have to concentrate on keeping the spell up what happens if the fire goes out prior to the end of the summoning? Would this count as loss of concentration and cause the elemental to go out of control?

Yes, you need to concentrate on it ‘as if you had cast the conjure elemental spell.’  The brazier replaces the 1 min casting with a 1 action command word, but everything else is the same Including concentration and duration.  The fire only needs to be burning in the brazier when you speak the command word.  If you lose concentration the elemental will go out of control, same as the spell.

Would you consider casting Continual Flame on this item as meeting the fire burning requirement?