Alarm fault on my Omega V6?

The remote will lock my Omega but more often than not it will not unlock it. When I then have to unlock the car on the key the alarm goes off. I have tried locking the car on the key and not using the remote at all, but when it comes to unlocking it (using the key) the alarm sounds again. I have changed the remote battery; no change, same pattern.
Startimg the engine obviously cuts the alarm off and then with the ignition on I reset the alarm system by locking/unlocking the car on the remote.
As I say, 8 times out of ten the alarm will go off when I have to gain entry to the car. This is really ticking me - and my neighbours off !
Any simple solution(s) I can try - other than the £220 quid an hour auto electrician or main dealer rates ?