Chose a 'Lloytron' wireless door chime with a plug in sounder on ebay - works well, all but two of the 'tunes' are so cheesy and awful they are embarrassing to play. So toss up between an electronic version of a knock at the door and ding-dong!
Bought the equivalent of a mercury switch from Maplins, seems to contain a ball bearing which makes the contact.
Dismantled the bell push and soldered the tilt switch across the contacts, adjusted the tilt, reassembled and used some double sided foam to attach the bell push to the underside of the mail chute.
Seems to work perfectly, now waiting for the next pizza flyer to test it!!