Well, bit of an update. I tried testing the circuits and found no problem. I have been onto Tracker this afternoon and spoken to a service engineer and he has assured me that my system is long overdue a service and that the internal battery back pack is only good for upto 5 years. Mine is 6 years old now. He is certain that the internal battery is dead and is now sourcing power from the car battery. So its booked in for the new battery backup pack next week. £80.01p!!