His insurers will accept or deny liability.
If the driver accepts blame/liability then that means his insurers are liable to pay
As the driver denies liability then his insurers will deny their liability to pay
But the incident/accident occurred and they would need to prove non liability
The witnesses earlier sightings of the other driver don’t directly make him to blame for the collision - but they would indicate the manner of his driving
Unless the witness saw the actual accident then they cannot state liability
Your partner isn’t an independent witness but can make a statement
Keep pushing back, try to get a copy of the statement the witness gave (or at least details of when and who to)
Add to that the photos you have plus diagrams etc
Ultimately you report and raise a claim with your insurers, they recover from his insurers
If you still are not getting anywhere then contact a solicitor who ought to give an initial free consultation & advice
……..
Many years ago when a van hit me & my passenger on a motorcycle, I reported to my insurer and pursued the van driver with a solicitor. The drivers company etc ignored all my solicitors letters, which didn’t worry my solicitor as he was content with the case and was just adding to their future bill.
(Note that my solicitor represented me, but included my passenger in the claim whilst advising that she may need her own solicitor to sue me which would then add to my claim)
The police prosecuted for dangerous driving, my solicitor gave me the court date and asked if I would attend, otherwise he would send a clerk and add that to the bill
I went, the driver sent in a written guilty claim, and then the companies insurers accepted and paid