1) Is date of death regarded as the cut-off point? ie income received before that date becomes part of the estate and therefore subject to IHT, but income received afterwards is outside the estate?
Income which accrues prior to the date of death is subject to IHT as it forms part of the estate of the deceased.
2) Should income received post death therefore be apportioned, if it partly relates to a pre-death period ?
For income tax purposes income due prior to death (even if paid after death) is the liability of the deceased.
Income due and payable after death is the liability of the executors as the income arises during the administration period (even if some it may have accrued prior to death).
3) Presumably the trust created for the administrative period is liable for tax. Is it taxed as an individual, with an allowance, or a trust, with a reduced allowance, or does it not have an allowance?
The executors are subject to income tax on income regarded as arising during the "administration period"; they are not entitled to personal allowances nor are they subject to income tax at rates other than 10% and 20%.In essence there is not "trust aspect".
4) I understand that funeral costs are a charge on the estate, not the administrators. Does that mean these costs offset IHT? What other costs can similarly be included - probate costs, solicitor's fees, travel, etc ?
Liabilities of the deceased subsisting at the date of death are deductible in arriving at the deceased’s net chargeable estate for IHT. These include reasonable funeral expenses and gravestone. They do not include the other costs to which you refer.
5) If not, can they be offset against income during the administration period? But not both, presumably?
The Apportionment Act is irrelevant for income tax purposes; it applies for the purposes of succession only.