Hi Ian,

We use -

Voilà! = Here you are!

often when you are handing something over to someone.

You would use -

Le/la voilà! = Here it is!

 

Les voilà! = Here they are!

to say you have found something

or of someone who is arriving 

Le/la voilà = Here/there he/she is ( there he/she comes)

Les voilà! = Here/there they are! (here they come)

Hope this helps!