First of all you need to write a spec. This shouldn't be a problem I did it all the time as contractor, and a test spec. Both of these don't need to be more than a couple of pages.
Then you need sign off, i.e. get your client to agree, that's what you are going to deliver.
Once you have that it is pretty water tight, because they have to show that the software contradicts the spec. When the customer is trying it on that is actually very difficult to do. In other words if what they're talking about isn't described in the spec, then it has nothing to do with the warranty.
Just wanted to point that out

One customer in the company where we did fixed price contracts refused to pay, so we took him to court but the customer had no documents other than stuff we gave him, and according to the contract unless he objected in writing within 7 days then he'd accepted it, so he didn't have a leg to stand on.

The other thing is warranty is with respect to bugs not "missing features" (remember the spec), and my experience is it rarely takes more than half a day to locate a bug, and then locate the "guilty party" in the code control system, and if you didn't cause the problem then simply bill the client for your time.
As long as you write a spec, test spec and get it signed off and software accepted you should be fine.
I personally would quite welcome the challenge, but I would attach the above conditions to the warranty and of course add some margin as well.
I think a contract of this nature would actually be clearly outside IR35, which is why I would like it.
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