Firstly I would like to say thank you to those that have directed me in the right direction with my alarm on my 2001 elite estate. It had a mind of its own going off when ever it felt like it at work and at home, which was p*?%ing off work colleagues and neighbours. A quick search revealed numerous threads on the power sounder and how they fail. £57 lighter today and now having fitted it, it is silenced and it appears that the range of the remote has also increased

?? Is this my imagination I wonder.
To fit the new power sounder, remove both windscreen wipers. If they are corroded to the the spindles, having removed the nut, pour boiling water over the wiper arms near the spindle and the soak in penetrating oil. Dont pour water over the windscreen, incase it cracks!! A firm wiggle and they freed off easily. Remove the scuttle panel and then turn the ignition onto stage two (all ignition/ dashboard lights lit). Remove the 10mm bolt and unplug the sounder. Wrap it in a blanket to deaden the sound or wear ear plugs. Plug new sounder in, turn ignition off and as a Haynes manual would say reassembly the the reverse of removal. Test the alarm by locking the car with the fob and then wait until the LED on the dash slows then unlock the car using the key in the door. Both the horn and the siren should sound.
Any way I have now dismantled my old power sounder with a view replacing the old rechargable batteries.
So for a 2001 Omega Elite Estate 3.2 the part number for the siren is GM 9194455
The replacement batteries for the sounder are Varta batteries part number 3/V150H with pins, you will require 2 costing you about £10
Follow this link [url][http://www.budgetbatteries.co.uk/20940/3-v150h-size-mempac-nimh-3-6v-150mah-varta-rechargeable-memory-protection-battery-pcb-mount//url]
To replace them carfully cut round the plastic join and separate the front and the back of the power sounder. Having got your batteries then desolder the old ones and solder in the new. The Printed Circuit Board is held in by four clips which can be released with ease. Having soldered in the new batteries reseal the unit using some instant gasket and insulation tape.
I now have a spare Power Sounder that I am keeping for when my new one decides to go belly up.