PhD student, issued contract at year 3 which will sign over intellectual property. Is it legal?
Legally, you can't be made to sign a retrospective, retroactive contract.

Professionally, if your supervisor or the university person responsible for the CASE studentships decided to be obnoxious about it, they could make the completion of your PhD uncomfortable.

As long as you don't need the CASE money or the on-site experience, this looks like a useless deal for you; in your shoes, I'd be minded to talk to my supervisors, then the faculty graduate tutor. Explain without emotion that you didn't sign up for a CASE studentship; ask if they can give you a good reason why you should sign up, because from where you are, it's hard to see any value in it.

They won't want to stop you finishing. You're EPSRC-funded, and having EPSRC-funded students fail to finish causes a problem for the department when it seeks a renewal of the funding. So you do hold quite a lot of the cards, here.

Ultimately, it's going to be a negotiation. It would be very surprising, and rather irregular, if they did sign up with the sponsor having given the sponsor the belief that this contract will retroactively apply to all current PhD students: I've only seen the terms and conditions apply to those students who start their PhD as CASE students. My hunch is that it's more that someone is just trying to get something for nothing - a free rider.