im in a similar situation, lost my only key to mine last week, its also got an after market clifford alarm on there.
i called out an auto locksmith who got me a key that works for the locks, but no immobiliser chip, as couldnt program one to it cos the alarm was going off. so i just disconnected the battery to stop the alarm going off for the meantime.
i am gunna have to try and get an alarm guy to come out and take it out properly then get the key guy to come back and hopefully programme an immobiliser chip to it so i can actually use it again. although he aid he might not be able to do it cos of the age of the car, and might have to send the immobiliser box off for a week with the new key. so im not sure what i would do if thats the case. all a bit complicated and bloody irritating! tax out at the end of the month and mot soon, not good.