Alistair late into this, I have a similar system on my 1999 Plus 8 but have never used it although I have two blippers for it which are as Rob's above. I have an original white label with blue lettering on mine AMR 4893 if that helps at all, the frequency is also on the label which is a little faint but will have a go at deciphering should it be helpful to you on a quick pass it looks like 483MHz.
When I picked up the car at Harpers Roger said, I wouldn't bother with those I never do, so took his advise.
In the seven years I have had the car there has never been a problem so I'll carry on, it's great big lump to carry around and I always throw the Red switch at home and away so safe as a house, and without the need for float charging the battery - 10 weeks without a run (holiday) at one stage and she fired first pot up.
