Thanks Ronnie, although I don't think that's the issue here. The problem was that the anti-theft device was still engaged so I couldn't open the rear doors to gain access to the box of tricks under the back seat.
Part of my problem now is that the weather is somewhat warmer than it was and it seems to play up most in the cold (i.e. sub zero C) - much harder to find the problem when it's not manifesting:(
As things have developed, so has my knowledge, so I have a clearer idea of what the issue is, and it may be that there are 2 seperte issues.
Effectively, when locking the car, the locks bounce. Turning the key a second time locks the car. However, if the alarm is set, several hours later, it will likely trip for no apparent reason (though again, this started with the onset of winter).
if it's really cold, then the vehicle will not unlock with the alarm fob, and turning the key in any of the doors which activate the central locking only unlocks that single lock and does not activate the central locking nor disable the anti-theft device, hence the rest of the problems. When the interior light was flickering, a relay could also be heard clicking under the backseat, and trying to start the car, the starter made the kind of noise you'd expect if the starter needed replacing. Seems like rather than the anti-theft being either on or off, it was flickering between the 2 states.
Gildersol's suggestion regarding a switch (locking or light) sounds quite possible as a cause here, which is why I was asking where the lock switches are located. Are they part of the lock mechanism? Is it something that can be tripped manually to test?
Again, thanks for all the help