Hello, I'm Keith and happy to help you with your question. I kicked off wrongly and am working on it. Inheritance tax (IHT) was introduced in 1986.
Checked info with firm of chartered accountants.
They indicate IHT relief to be transferred to mum's is circa £35k/£71k x £325k =£161K.
So added to £325K = £486k.
£35k is the only bit of the intestate sum of £110k estate that was not passed absolutely to my mum. In fact 50% of the residue was in trust for her so the only amount that actually passed was £17.5k. Therefore only £17.5k of the £110k was non spousal transfer.
Could therefore expect IHT relief transfer to be £71k-£17.5k/£71k x £325k = £240k approx = £565k.