engine immobiliser won't turn off?

long time listener. first time caller...

folks, i have a 1990 vitara jlx. have had no problems until this morning. i turned off the engine and the alarm and immobiliser kicked in for no reason.

So I turned off the alarm with the fob, but the immobiliser is still on. No amount of playing with the fob will solve it (the fob still activates/deactivates the alarm) and I have even gone under the bonnet, disconnected/reconnected the battery and wobbled things a bit. y'know the standard things you'd expect from a bloke who hasn't the faintest idea what he's looking at or what to do...

Before calling out 'the man', is this a standard fault that has an easy solution?

Thanks for any assistance guys.

G