Transfer of spouses allowance

[Craigross]

This is a bit of a strange and complex question. Both of my parents passed away last year. I am the executor of the estate and am currently dealing with probate.

My question is as follows. My father doesnt appear to have applied to use my mothers unused tax allowances. Is it possible for me to make the application and have it back dated?. Or is it something that is only applied when asked for

[pawncob]

You can claim it for 2015/16 and previous year.