Hello again!
I've got out our invoice from having the chimneys lined and it cost £2,000 to have two chimneys lined, including labour and all materials. The liners themselves are not the expensive part. Once the liner is in place, the rest of the chimney has to be filled with vermiculite (a bit like cat litter) to provide further fire protection. Our chimneys are moderately wide (I could probably wriggle up them if I had to) and we needed 16 bags of vermiculite costing a total of £300.
However, on top of the £2,000 we had to spend a little under £1,000 on scaffolding, because obviously you can't crawl around on a thatched roof like you can on a tiled roof. The lining job itself only took a day and a half, with just one chimney sweep, but the scaffolding stayed up longer because it was provided by a separate company on a different schedule. We also had to have nearby electrical lines sheathed in case the chimney sweep touched them and fried, but your local electrical power will do this for free.
Yes, you will definitely need fireboards underneath the roof. Like the chimney liners, these aren't massively expensive in themselves, but are a bit of a faff in terms of labour.
If you are not being pressured to make an offer on the house, I highly recommend that you contact the Conservation Officer local to the property. They will need to be your best friend if you are planning to do so much as change a lightbulb! You can chat to them informally, tell them your ideas for the property, and ask about any grants that are available for the work you plan to do. It's the best way to find out whether you are going to be able to make the house as you want it to be without encountering issues with the Grade II listing. We actually pulled out of a property after making an offer because we spoke to the local Conservation Officer and had every single idea for renovation turned down.
The one other thing I wanted to warn you about is that you will almost certainly have to insure your house for more than its commercial value. Our house would cost £100,000 more than the sale price to rebuild because the materials and construction methods have to be authentic. This can make your insurance costs even more eye-watering! You need to ask your surveyor to provide a rebuild value, not just the drive-by value usually required by a mortgage company. Rebuild values are calculated on the size of a house, but bizarrely, the size of your house will not be the sum total of the room square footage. It has to include the walls as well, and in the case of a thatched cottage with super-thick walls, these can take up several hundred square feet.
I will stop hurling information at you now and leave you in peace, but please ask if you have any other queries. I am not an expert by any means but I have had a crash course in thatched house/listed building ownership, and learned some hard lessons!