Hi Alistair,
I have a 1997 +4 with an immobiliser factory fitted and indeed it has the same black box under the dash as you mention.
The fob that I have has the number LUCAS 3TXA KEY on the back, it is a black fob with 2 buttons, on the right hand side button there is a raised decal of a lock.
I have put the part number into Google and it seems to recognise it.
Hope this helps in some way.
Rob...
