Sigma immobiliser preventing engine start, how can I fix it?

We had a Sigma immobiliser installed on a Chevrolet StarCraft Day Van probably close to 10 years ago (unsure of model immobiliser). 6 years ago the vehicle was declared off-road and has been sitting on a drive-way ever since. The batteries had died and have since been replaced, but now the immobiliser seems to be preventing the engine from turning over. The paper work has been lost, we cannot remember the codes and the garage that installed it closed down a few years ago. Is there anything I can do to remedy this?