alarm Problems on a 1997 rover 216?

Recently the battery ran flat on my rover 216 and I had to remove it from the car to charge it.
After re-fitting th battery the alarm went off constantly unless I disconected the battery again.
The key fob did not work and the alarm lights atayed on.
Is there any way that | can cure this problem without having to pay ex rover dealers over£150.00 to fix it
Kind Regards
Paul Austin-Clarke