540 Alarm reset, anyone managed it?!

Following a dead battery, my 2nd 540's alarm goes off when I now connect a new battery to it.

I've followed the instructions in the owners manual -

Shut drivers door, turn key in ignition to position 1 and leave for 15 minutes. Horn stops after 30 seconds, hazards turn off after 5 mins, but the alarm LED will no go off even after an hour! Any attempt to switch ignition on, or open the door sets the alarm off again.

I'm guessing I need to re-synchronise the remote key as it won't stop the alarm, but I guess that won't work until the alarm is deactivated! Catch 22!

Has anyone successfully managed to deactivate the alarm?

If tried following the suggestions I found when searching other posts here and on the net, but none seem to work :( .

Car is a 94 540 btw.

TIA