"There is no panic button on my key fobs, I have three two plus a service fob, not sure if these are different to the American and European spec?"

I learned something here; The European fob has no panic button and it also works on a different frequency than the American cars. SO to test your siren you can get inside the car, lock the doors with the fob and wait until you see the red light flash on the tow alarm switch ( I am assuming you have that function, as it indicates the anti theft alarm is active) then open the drivers door from inside the car. Your siren should start blasting. Read your owners manual as to how to turn it off before you do this test. Here in the States we can simply push the Panic button. You may have to insert the key into the ignition, or from the outside insert and open the door again with the key. Check your owners manual.
A siren that does not function can cause additional problems. It has internal batteries which over time leak and ruin the circuit boards which are part of the anti theft system. You may simple disconnect a bad siren (it will do no harm that way) or replace it with a new one. That choice is yours. Mine still works on the SRT6, but it would be the exception. Many of the originals have failed by this point in time.

As to the water in the boot. You will not find dampness there. The water from a leaking wing will run down behind the panels and find it's way to the very bottom of the rear of the car. there is a compartment under the trunk carpet that fills with water and ruins expensive modules. If you are having no trouble you should consider putting a hole in a rubber round plug that is under the car's rear just outside the exhaust pipes to allow water to drain out of the compartment. Please use the search box in the upper top corner of the forum for water in trunk so you are informed of the problem and are aware of the remedies.
Good luck.