I have found the solution in my Mito. This is how I did it.
Take an old pair of jodhpurs (made of a sort of thick, soft felt material).
Find the siren (open the bonnet and keep pressing the key button, locating the siren with your ears).
Undo the one screw on the right hand side (looking from the front, so it is actually the LEFT (passenger side) of the car) of the back black cover over the alarm siren and pull out the one plastic clip. That is enough to gain access with your hand.
Stuff the jodhpurs all round the alarm siren, including underneath it.
Replace the clip and screw. THE BEEP OR BLEEP OR SIREN CANNOT BE HEARD from more than a couple of feet from the car.
WHOOPEE - IT WORKS.
P.S. Steve, apparently the 'expert' at Johnson's Fiat and Alfa in Dorridge near Solihull told me that it couldn't be done or if it could be done, he wouldn't do it in case it was illegal, but it is up to me what I do with my own car.
Last edited by Chrisikins; 17-07-14 at 22:12.