Hmm, tricky one
As above, you need to handle different lenth area codes and the possibility of a dash, space or not between the area code and the number...

Also, you need to drop the leading 0 to put the 44 on the start.

So maybe something a bit like this...(Using a php base for the pseudo code)

$cleannum = preg_replace('/0?([0-9]{4,5})[^0-9]?([0-9]{6,7})/', '0044\1\2', $num)

HTH.

Edit: Sorry, didn't spot it had been solved a lot more completely by that HUUUGE monster above!
(MrOli is ugly, but in the morning you will be sober, Sun 7 Jun 2009, 14:38, Reply)