Thanks to this forum found the problem with my alarm sounding willy nilly and again with help from forum member received instructions on how to correctly remove.
Well having removed the unit, cut it in half to have a peek inside to see how bad it was. Therein came the surprise, the batteries are totally different to what I was expecting to find, instead of the green type there are two large button type cells marked Z3A 120 and indeed they had leaked, but not over the circuit board. Have looked through forum but not seen this type before hope it may be useful to someone to know different types are out there, check before you order.
Pics are here:
http://www.flickr.com/photos/28055763@N03/