@Grainyawaydays - I'd suggest you put to one side the suggestions to contact the police, or to call the court to see if the claim is genuine, or to start looking into it yourselves. No doubt they're well-intentioned suggestions but I'd be concerned that they might compromise your husband's insurance cover.
ALL that your husband needs to do at this stage is (1) to inform whoever was his insurer at the time of this alleged accident that he's received a claim, and (2) pass ALL current and future correspondence relating to this claim to his insurer at the time. (It might be the same insurer that he has now or it might not. you haven't said).
He doesn't need a lawyer, he doesn't need to start investigating it himself and he doesn't need to meddle in his insurer's handling of the claim. (They are professionals and don't like paying out fake claims. They are experts at recognising and investigating them - you and your husband are not. Don't jeapordise the insurance cover by doing anything more than letting the insurance company deal with ALL of it).
Your husband must have been paying insurance premiums for years - it's exactly this service he was paying for. Let the insurer deal with it and don't interfere at all.
NB - You and your husband are no doubt concerned that the claim is issued against him personally. If he is the person that the claimant is alleging caused the alleged accident then of course he's the person being sued. But the whole point of a motor insurance policy is that if you - personally - get sued, then your insurer steps in to defend the claim on your behalf and you don't need to stress out about it. That's what an insurance policy is. What you don't do is not inform your insurer, lose the case, and then try to claim on your insurance. You have to let them deal with it from the outset