I'm not British, Chrissy, but this will move your question up to the top of the menu again.
As an isolated date (as at the top of a letter) I would use only 2
April 2005 (April 2, 2005 is beginning to look quaint). In text
(i.e. in an essay, etc.) I would use the same, or any of the others--
with appropriate commas, of course:
April 2nd, 2005
The 2nd/second of April, 2005
April the 2nd, 2005
The second of April in 2005