Hi, I'm new to this forum as you can see. I have a question for you guys.

My b2500 has developed a fault after sitting on the drive for a few months.
I couldn't get it started and it seemed like an immobiliser problem, so a friend advised me to reset it by connecting the battery leads together and leaving it for an amount of time. I left it overnight and it started fine. However, it cut out whilst driving one day and wouldn't start. I ran the battery down trying to start it.
Once I charged the battery it seemed fine, but the red "key inside the car" light (which I assume is the immobiliser) flashes even when the vehicle is running. It flashes slowly, then stops, then flashes quickly, stops, then slowly and so on.

The vehicle seems to be running fine now but I'm scared to drive it anywhere incase it happens again (I use it to work on the beach so if it cuts out I could lose it in the tide).

How can I find out what is causing the light to flash in this way and weather it's going to affect how the vehicle drives in future?

Thanks for taking time to read and thanks in advance for your help.