The total value of your stepfather's estate would be £309,000 plus £360,000 plus the value of his part of 8 acres.
As for the house, there is a residence tax nil rate band, which is currently £175,000. If he inherited the house from your direct ancestor who did not use his/her residence nil band, then the amount of £175,000 can be doubled for you and the total of £350,000 will be deducted from the value of the house for inheritance tax purposes. The condition, however, is that the house is passed on to your stepfather’s lineal descendant (like you or your child).
In addition, a deduction of nil rate band of £325,000 will be made from the value of the whole estate. So, the total deduction you will have is either £500,000 or £675,000, depending on how your father inherited the house.
To answer if his part of 8 acres will be taxable, please specify if he owns it with nephews jointly or in common? If jointly, his part will automatically pass on to nephews upon his death, if in common, this property is taxable and you need to add the value of it to the value of the house £360,000 plus cash £309,000. Then you will need to deduct either £500,000 or £675,000 and the tax will be 40 % on the remainder. It may be paid in 10 instalments.
Hope this helps. Please feel free to ask further questions.