Well just about everybody in our family says....

"A-ha", she cried as she waved her wooden leg to the postman.My Nan and her generation used to say it so it followed to my mum and me and so on.

Just one of those phrases that makes you chuckle along with...

'daft as a bucket full of a**eholes.','fetch a copper','you must think I came up the Thames on a bike' etc.

he he

great thread of answers and interesting to see all of the potential sources. I guess from this that the phrase has been around for a while.