Donald,
I have just re-read your original post and notice the following -
"Putting the values from these schedules (IHT403, IHT409 and IHT411) together with the small amount in her bank accounts (IHT406), a small amount of Household and Personal Goods (IHT407) and Life Assurance Policies (IHT410) is giving an inheritance tax liability (in box 119 the foot of page 12) as £8,400.
As her whole estate (her 44% share of the house at today's market value, plus the amounts in her bank accounts, the value of her shares and her personal possessions) totals around £240,000, so the inheritance tax figure of £8,400 is obviously wrong..."
In your first paragraph you mention Life Assurance policies and in the second one you don't. Unless written in trust for someone else on death, the value of Life Assurance policies form part of the Estate. As I previously pointed out to you, the IHT estimate of £8,400 equates to an estate of £346,000. I don't suppose the Life Policies were worth £106,000 were they?!