Inventory of a house for inheritance tax purposes?

[worktrip]

Have been asked to do this. Do they want me to list every item and work out its second hand value? Like every pen, spoon and tweezer?

No you don't need to do that.
If there are specific items of value e.g. original work or antique furniture or collections then you should get those valued.
But the day to day contents ultimately have little value. It's not the same as contents insurance value where you would be looking for replacement value.
The valuer from house clearance gave value of £500 for the furniture, clothes and general 'stuff' in my parents' 3 bed house after Mum died last year.

Original artwork that should have said

I had to do this last year and apart from valuables like jewellery and antiques I was told to value items as if I was selling them on Facebook selling pages. The whole house came in at £600 and wasn't queried.

Get a house clearer in to value it

thank you! Was totally dreading it!

As above. We valued contents of my sister's home at £500 as that's what we'd have got second hand. As long as you don't take the piss with car valuations etc, HMRC aren't bothered.