Transfer of unused inheritance tax nil rate band

My dad passed away 10 years ago however the was no will or formal executor. The only asset he owned was 50% of the house with my mum. After his death we transferred the house into my mum's name with land registry. No forms were sent to HMRC. The house is worth close to £600k, in the event my mum was to pass away and leave the house to my brother and I, would IHT be due on her assets above £325k or can we claim husband and wife allowance £625k. Is it too late now its been more than 2 years since my dad died? Any advice? Thanks

Hi,
Please refer to:
How Inheritance Tax works: thresholds, rules and allowances
You can also contact our Inheritance Tax team here:
Inheritance Tax: general enquiries
Thank you.