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I was thinking more along the lines of "knock for knock" (although I said 50/50, on reflection I didn't mean that).
My assumption is that this person cannot prove that we caused damage to his car, any more than we can prove that he [may have] caused damage to ours.
Therefore each insurer responds to their policy holders claim (we don't intend to make a claim over a scratched bumper).
£500 excess and £600 of damage
They succeed with 50/50
Your insurance pays £300 they pay £300 as it’s within their excess.