IHT—am I overthinking it?

I'll try to keep it simple but I'm totally confused after HMRC have changed their minds about what forms I need to complete!

Mum died, left no will. She was long divorced and I'm her only child, so everything will go to me, correct?

The estate consists of a house that's unlikely to fetch much due to it's state of disrepair (anywhere in the region of £100K would be an absolute miracle, I reckon). She had a little under £10K in the bank. There is one item of jewellery that *may* be worth >£500 and one piece of furniture that's the same (not that I can sell it because it's hard wood with no provenance...). She had a collection of teddy bears I valued using eBay listings but aside from the odd decent electrical everything else was either broken or shopping channel tat. Most of that has already been dragged up to the charity shop and I could only guess at the value of it—decades old kitchenware, clothes that probably came from the shop we took them back to, ditto books, and general bric-a-brac of virtually no real value—they'll be selling it for pence or a few quid at most! She also had an over 50s life insurance policy I'm told was worth just under £10K.

I started filling in IHT205 as advised (I told all of the above to the Probate helpline and they said it'd be fine), but when I got to the question about life insurance it says I must complete IHT400. I rang Probate again and they confirmed this was correct. I started looking through it but as I was told I wouldn't need to put a value on everything I was giving away, I have no receipt and can only guess the value of the stuff donated. The estate isn't worth anywhere near the IHT threshold of £325K so this all seems like an unecessdaff. And I can't provide the documentation the schedules request anyway! i was hoping to save money by doing Probate myself but it doesn't seem as simple now HMRC want an inventory and value of everything—even if they'll accept "furniture" or "clothing", I still don't have any idea what to put, and all the clothes, books and kitchenware have been donated or binned now, so I can't make an accurate list! A small part of me thinks I'm really overthinking it, but I'm scared of getting it all wrong and them trying to argue that I've given away a Picasso painting she had hidden under the bed or something, simply because I can't prove that I didn't (I didn't, obviously, but I couldn't prove it)!!

Help, please. The forms don't look that hard, I'm just not sure I need to be filling them in or what to put re: value if I do. So I need a solicitor for an estate that's worth about £100k including the property, really?

Thanks