Intermittent immobiliser light flashing 1-3 on Ford Fiesta Azura 1994

I wonder if anyone could help me, I have a dreaded intermittent problem with my 1994 Ford Fiesta Azura.

Sometimes when I turn the key to start the car, the red light under the clock starts flashing and the car will not start.  The lights, clock etc are working and the battery has the correct green circle.  I did a bit of googling and found out about the PATS system.  If the light is left for a minute it flashes in a pattern of 1-3, which apparently means - Key code not received is the key obscured, try again with different key, key may be damaged/faulty.

I have only ever had one key since I bought the car second hand in 1999.  I have a black key with a red dot on it.  Apparently there should be a red master key but I have never had that.  I didn't even know the car had an immobiliser.

When this first happened I got the car towed to the garage who could not reproduce the problem because, of course, once they turned the key it started at once.  So they checked all connections and found one thing that looked dodgy so replaced that, but the problem still occurs intermittently.  

Googling suggests disconnecting the battery and leaving it for 20 minutes, which I have done when it happens, which isn't ideal but at least it's a fix.  Sometimes I turn the key, light flashes, I turn it off then on again, and it works.  Other times I only leave the battery disconnected for a couple of minutes, try it again and it works.

However the other evening it happened again and leaving it for 30 minutes with the battery disconnected didn't clear the error.  I was about to phone the AA when my husband switched the key off and on three times quickly - the light came on and stayed red, so he quickly started it and we got home.

More googling suggests things like changing fuses - I don't think it's that as it would then not be an intermittent fault.  The evidence seems to point to some kind of loose connection but as I said the garage has already checked for this.

The problem doesn't have any pattern as to when it occurs.  Obviously it only happens when starting the car, but this can be after the car has been sitting in the garage for a few days, or after it has completed a journey with no problems and I am starting it up to return home.

The car currently has no heat sink (a sort of protective panel thing under the engine) as it came loose and the garage said it was not necessary.  But could the lack of a heat sink be affecting it and allowing damp to get to places where it normally wouldn't?

Please could someone suggest a course of action to take or anything to investigate.  I know the car is old but it works otherwise and it seems a shame to scrap it because of this.  I need a reasonably reliable car though.

Thanks in advance for your help