Further to the above - did as suggested, hooked it up wothout actually installing.
Also found a surplus to requirements 240v to USB plug. 
There are 60 different tunes to use for the 'chime' - have to find one that does not irritate - as when the button is pushed [ and it must work by WiFi or similar] the tune plays its 5 seconds or more !
Modern technology!
Thanks guys.