Siren keeps going off,have electrician looking for siren and battery is it under windscreen wiper or wheel arch,thanks in advance from a brand new member,so apology if not clear,I am very not technical

As standard there is no siren or back-up battery. The alarm uses the normal horn.

2 hours ago, Llanigraham said:

As standard there is no siren or back-up battery. The alarm uses the normal horn.

No it does not and it does have a back up battery that needs changing at about 5 years.
Set your alarm off and see if it's coming from car horn.
I think it's under the back somewhere.
My motorcycle has similar and that takes a NiCad PP3

I stand corrected, but the only time my alarm has gone off it definitely sounded like the normal horn. 

1 hour ago, Llanigraham said:

the only time my alarm has gone off it definitely sounded like the normal horn. 

My dual tone horn is melodious but alarm is Very shrill.

 

I would have guessed under the RH front wing above the arch liner like the Octavia but having removed my wing recently to repair it the alarm siren definitely was not there but I didn't realise it till now.

 

So I can tell you that its not where it was likely to be and like you I now want to know where it is, only a matter of time before I have to replace mine.

In service schedule alarm battery to be replaced every 6 years (but I doubt if many are), My Fabia's was still working well at 11 years old.