Proof of identity form?

My mum died 10 months ago and left my sister and I her house in her will. My mum was the sole owner of the house. We have the Grant of Probate and I now want to transfer the house into our names; I understand I have to fill out form AS1 but do I also have to fill out a proof of ID form? I feel certain when I rang The Land Registry a few months ago I was advised that the Grant of Probate was proof enough as it had both my sister's and my name on it. Could anyone confirm this?

Also, is there a time limit to when we have to complete the transfer?

K P - Forms AP1, AS1 and ID1 needed - see our online guidance 
Our identity requirements altered in September so the executor(s) now need to have their identities verified 
There is no time limit

Thank you!