@Alex75, it's an unfortunate incident, but given that the knife was already 6 months old, an offer of 70 GBP might seem to be reasonable. You could, I think, escalate the process by going to the Resolution Center, but maybe the simpler/wiser choice is to take the amount offered, and look at this as an AirBnB learning experience. One thing that new - and even experienced - hosts are always advised is not to leave out anything of value that could be damaged or stolen.
I'm more concerned, actually, as to how your guests were able to stay an extra night at no cost. I presume this was arranged outside of AirBnB, which means you wouldn't have had coverage for that period if anything disastrous had happened. Seems to me you got off lightly with only a damaged knife and the offer of partial reimbursement.
I note that you have no Security Deposit required, and have very generic House Rules. I strongly recommend having a Security Depost, as you can claim against it for damages by future guests. I also recommed creating a very firm and no-nonsense set of House Rules.
In any case, perhaps other hosts here will advise you differently with regard to the knife.
But don't be discouraged about hosting. Once you set up the boundaries and put certain measures in place, it's a lot of fun. 🙂