hey funnysambam,
you carry on helping were you can, don't be upset by others here, we are all here to help the DIYers het there alarms fixed, or point them in the right direction,
in regards to the original poster.
if the rear external siren unit is the live, and you cannot access it you can do the following to check whether the siren is causing the fault.
remove cover on your control panel
remove the T and A wires from the panel and the D B (remember were they went as well) and put a cable link between T and A (do not put a link between D and B (you will blow a fuse in the panel by doing that)
put the panel lid back on and try to reset the panel.
if the panel resets, you have found the fault
if it does not, i am sorry but i am of the opinion that you panel has died.
(make sure that the battery is a GOOD one, before doing any of the above)
and be careful of the MAINS SUPPLY in the panel!
hope this helps you.
PS
this will tell you what to tell the engineer, if the external siren is faulty, you should only need a new siren and not a new system.
also the engineer should check the charging circuit on your panel, especially the load resistor on the PCB, some times if the battery is so old it will not charge the load resistor will fail and cause a large black mark on the PCB, that will mean you will need a new panel!
Oasis