key fob?

i recently disconnected the battery on ASTRA MK 4 SRI whilst changing the alternator.
the key fob would work properly before this but once put back together i can not open the car on the fob.i can still open it with the key but the alarm goes off ( factory fitted) until i switch the ignition on.the car still locks on the fob.i have changed the battery in the fob and tried retuning it by switching the ignition on and pressing the open button on the fob but it still dont open.HELP

neal
June 2009