I’ve also got an R I’m due to receive, with the funder also being Arval. I signed up for this back in November, with a different broker. My experience with Arval (and I’ve just double checked my inbox to confirm) is that the paperwork is sent to you for electronic sign off and once you’ve signed it off you’ll receive an email with the completed paperwork attached.
So, assuming you have done the paperwork you’ll be able to prove it with that email. However! When I was sorting this out back in November, I had to chase the broker to get Arval to send me the finance documation. Assuming this was not a broker specific problem, what you may find is that the docs you remember signing were only for the broker agreement - if they failed to send you the finance paperwork unprompted and you didn’t chase them for it then you may never have put the formal finance agreement in place.
Like Ken I say, check your emails for stuff from Arval because if you did it it’ll be there.