Immobilisers (a bit long)

Excuse the background, but I'm always complaining about posters who never provide enough detail.

I have a second-hand ten year old ATV (Cub Cadet). It came from the Surrey/Hampshire border, in the stockbroker/pop-star belt and came with an (apparently simple) Sterling Touch immobiliser. I've been using a detachable battery terminal because there's a bit of battery drain, and sometimes I don't use it for a month. Recently this terminal has been a bit flaky so I decided to clean up the terminals and put in a "proper" isolator like this (except that I think that mine is rated 300A, which should be OK for this 300 cc diesel).

Unfortunately, after doing this at the battery end (the terminals were very corroded and needed a lot of cleaning up) I found there was no power getting through to the isolator "touch point" on the dashboard (which should "flash" when it is disabled even with the ignition off). Also the glow plugs and the electric fuel pump don't kick in when you switch on the ignition, as they normally do, provided the immobiliser is disabled. (There's a glow plug light).

So I expose the back of the dashboard so that I can see if power is coming through to the ignition switch, etc.

After taking out the radiator to get access, I discover that I have a full blown Thatcham-rated aftermarket alarm and tracker system suitable for a Lamborghini, not just a simple immobiliser. This includes a box with an IMEI (so presumably used to be able to "phone home" on theft), plus an alarm sounder, plus (by the look of it) an FM transmitter for the police to track. All professionally fitted to a high standard, as far as I can see. It's quite difficult to see what might have been the original wiring, and which the "aftermarket". The "touch sensor" seems to be connected to a relay which is connected by fairly fat wires to the back of the fuse box. It's a diesel BTW (Japanese "Caterpillar", fairly "low tech"), with an electrical fuel pump that supplies the mechanical fuel pump.

I suppose the immobiliser could disable the starter solenoid, or the electric fuel pump, or the glow plugs, or enable the engine cut-off. The Sterling Touch web site suggests they currently provide two lines of protection, perhaps the starter solenoid and something else.

The vehicle is still worth £5k or more if only I could get it working, and lives behind locked gates at an unoccupied but not isolated stables, but really doesn't need all this protection. But of course it only has scrap value unless I can get it running again.

Do you think I need to go to an immobiliser specialist (there seem to be a couple withing 20 miles, but I'd need to get it trailered to them) or might I be able to unravel it myself? If so, where should I start?

It lives outside, so connectors and spade terminals are beginning to get a bit corroded. I suppose this might actually be the problem.

TIA!