Help! Subaru Legacy 2011 car disabled by immobiliser!

[Philip JS Morgan]

Hopefully someone wise will point me in the right direction please....  UK Legacy estate 2011 (last production version...) . Key fobs ( both) won’t lock or unlock car, so have simply used key . All fine. For second time in about two months, I come to unlock car, open door, and it seems to have set the immobiliser on its own...the alarm siren goes off , plus indicators etc. Don’t know , but might be the Battery is low. Not used much in this COVID lockdown! 
But now the car is disabled and won’t start, just p*ssing off the neighbours every time I open the door to try and sort it.

So, how do  I disable the immobiliser please, given the key fob doesn’t work. ? 
car is now totally disabled! 
have tried turning on and off 3 times with key in ignition...alarm still making me deaf.

If I could take the whole alarm system out and chuck way, I would...

thanks in advance for any words of wisdom