Hi -- You may be filling in the wrong declaration form, the forms on which you'll need to record the valuation will differ, depending on the expected valuation amount. You complete a form IHT205 (C1 Scotland) for estates where you don't expect to have to pay Inheritance Tax (called 'excepted estates')
Also if the property was jointly owned by your Parents as "joint tenants" rather than "tenants in common" it passes automatically to your Mother, without becoming part of your Fathers estate at all and does not need a valuation, as it's a non-declarable asset.
The other assets have to be valued as a rough estimate (note you do not have to pay just give the value based on what you would realistically expect them to fetch at sale) of their true market value, but if the total is under the current IHT threshold (£312k) it's only a formality and will not attract any tax liability. However, it's a formality that is a necessary legal requirement before probate can be applied for and granted by the courts....which then gives legal control to the executor(s) for disposal etc.
If you are struggling then get a reputable house clearance firm in to give you an offer on the items in question, ask for a written estimate of what they are prepared offer, this will suffice as evidence towards their true reasonably expected value.
Don't forget any valuables that they both owned, you only need include your Dad's half the value on the valuation form, as the other half is already owned by your Mother, it's not her share of assets that you should be declaring. !
HMRC can, but rarely demand a professional valuation, except in cases where the valuation is just under the threshold and they have reason to believe items have been grossly undervalued, to evade any IHT liability that otherwise would have been due. !
I enclose a link that is useful for you below.
http://www.direct.gov.uk/en/Governmentcitizensandrights/Death/
BenefitsAndMoney/DG_10029819