Hi, I'm really needing some help. My superb 95 540i Tourer (Oxford Green/Buffalo Leather Sports Seats/Staggered M Parallels) went off the road last winter and has been laid up since. Three weeks ago I decided to resurrect the car. The guy who was storing it for me has lost my keyfob (which was the black "one button" rectangular shaped one with the corner cut away) although he has my key. However, I do have a spare key and sigma "one button" keyfob which is damaged (having a gaping hole just where you need to press it).

Anyway, to cut a long story short, I put on a new battery, tried the damaged sigma remote (with bluetack in the hole), the doors opened and I got in and started the car up. I was delighted given the amount of time it had been lying there and the fact that the spare, and damaged, keyfob had worked. I ran it up to temp, took it for a spin and then parked and locked it using the keyfob.

I noticed that the alarm siren sounded like it was really struggling to make noise, like it was on its last legs.
I tried the remote the next day but it didn't work, with no dash led feedback whatsoever.
I then started researching Bimmer Forums and found the great Alarm ID Guide by BillyWhizz
I identified that I have a Sigma (although branded BMW) 3T siren which I took apart and saw that the backup battery had leaked over the PCB.
I sourced another rechargeable backup battery and connected it and now the siren sounds (for a few seconds when I connect the main battery) but the central locking/immobiliser deactivation still does not function.
I then changed the sigma remote keyfob battery to see if that was an issue (now I have a slightly brighter LED on the keyfob) but still no response from the alarm system.
I have also tried disconnecting the alarm module under the back seat, to no avail. I then reconnected it, also with no luck.
I'm now at the point of going slightly mad about this. I know I could have an autoelectrician remove the alarm but it worked really well with full closure and I'd rather repair it. There was never a problem with the car like electrical drainage. The dash lights are strong and bright but I have no feedback from the alarm LED. I'm obviously opening and closing the car mechanically with the key.

What does the PCB in the 3T siren actually do? Has this got any bearing on why I have no LED feedback from the dash?
Is there any way I can deactivate the immobilisation system myself to get the car in for an MOT while a sort the alarm problem? (I have tried opening and closing doors twice in succession etc. . . etc. . .)
Can I simply put a new 3T siren in (if I can find one), or are the electronics in it specific and coded to my alarm system? (If the Siren module is the problem).
Could the main alarm module be faulty?
Could I have blown a vital fuse when disconnecting/reconnecting the car battery?

HELP PLEASE, ANY IDEAS? I'm desperate to get the car in for an MOT and get her back on the road. She's my pride and joy.