Immobiliser deactivation or correction: How?

Similar problem to previous question on 1199 Ford Mondeo 1800. New batteries would not hold charge. Sympathetic garage replaced two. On third occasion when service garage (different) had done full check and affirmed no electrical problems, the car failed to start. Obviously a fuel problem. Mechanic certain the fault is with immobiliser, which may have caused other batteries to run down after two days parking. How to confirm immobiliser is faulty and how to fix?