Fixed at last! Got a new siren/control unit off eBay different part number but just the tone of the beep seems slightly different £80 as opposed to about £200 from Fiat, didn't want to risk £200 on something that may not have fixed it! The reason for failure of these units seems to be neither battery failure nor damp. The unit is sealed against damp but this has the side effect of holding in very small amounts of corrosive fumes from the charging of the batteries and this corrodes the circuit board. That's the theory I found on the net and seems plausible