Problem solved!!!
It turns out that the alarm horn has an internal battery - which is prone to leaking all over the PCB inside.
Took me ages to find it - It was tucked up by the nearside A pillar - behind the ECU plastic box. The only bit you can see is a bolt, the horn itself is completely hidden.
Had to remove the scuttle panel, both wiper arms, the windscreen deflector panel and finally remove the lid of the ECU container!
Replacement off ebay was £16!!
Mark