Is this a scam?

Ask the supermarket to look on their CTV.

I thought of that but it's a fairly big carpark and I doubt it's all covered by CCTV.
If the insurance inspection (which took place last week) says we do have a recent scrape that matches the allegation, I might give the supermarket a ring.

Ask to see the third party photographs?

That's the first thing I did. I was told by the insurer that they weren't available to us

I would think they should have independent witnesses, I know when someone hit my sons car even though she put her hands up right away the company wanted to have someone else who had seen the accident.
Otherwise anyone could claim someone had hit their car, and anyone can take a photo of a nearby vehicle.
Very suspicious I would say, you need much more proof and don't agree to take the blame if they don't provide it. Your insurance company should be asking more questions and not simply expecting to pay against your policy.

jusnoneed, my suspicion is that the "third party" who took photos is the other car owner's friend...

Do you know what time they allege the accident happened? It may be your DH can prove he was somewhere else at the time.

This sounds fishy to me. I would definitely press the insurance company for evidence of the alleged incident.
The following is from Insurance Business UK.

“Fraudulent car insurance claims are on the rise, as opportunists try their luck at making exaggerated claims after a genuine accident or reporting injuries to phantom passengers, which have a knock-on financial effect on innocent customers,” said Mark Chiappino, counter fraud manager at Churchill.

Here is a breakdown of the top methods used for fraudulent claims according to the research:
Low-speed impact (when both vehicles collide at low-speed, such as in a car park): 36%
Organised fraud rings: 15%
Phantom passengers (when the claimant was not in the vehicle at the time): 8%.
Induced Road Traffic Accidents (when a motorist deliberately causes an accident with an innocent party, such as by slamming on the brakes): 8%.
Staged Road Traffic Accidents (cases where both vehicles are complicit in a fraud): 7%.
Exaggerated loss/ damage (when an accident occurs and circumstances are exaggerated): 3%.
Indemnity/policy issues (based on a lie when the customer takes out a policy): 2%.

It seems that what the other party is claiming is top of the fraud list.

My nephew was in a similar position but the other way round. His car was scraped, the person he accused insisted he was not responsible.
My nephew was initially told by his own insurance company that there wasn’t anything they could do. He was ready to give up. But his wife did a bit of digging and found they did have to help him. So they argued with their own insurance company.
An expert was sent to examine both cars and my nephew was proved right. Paint and position of the damage on the cars proved it. Please insist your insurance company fully investigates.

Certainly sounds like it Sarah, and if they have photos that show your husband doing the damage why won't they show you?

Been there, got the t-shirt. My OH apparently scraped another car coming home from work, he knew nothing about it, but when it came to me renewing the insurance for my car with him as a named driver I was informed that he had been in an accident that I hadn't declared!. This came to light because I was disgusted with the amount my insurance company had increased and I was looking for a better deal. My OH contacted his insurer to find that they had indeed known about his 'accident' and had paid out on it, all without even letting us know that this had 'supposedly' happened!!! Their excuse... that he was covered by fully comp, no claims, so wouldn't affect him. Sadly it did affect both him, as he has motorbike insurance with another company and also me, as he is a named driver on my insurance. Believe me, this is a short edited version of what we went through, they have no care, no sensibility, it all comes down to money, money, money.

shandi6570 what an ordeal!
Thanks all for your wisdom.

Unless they have a photo of the actual impact, and the other car was stationary (no one at the wheel) at the time, there is no proof of anything. The photos could have been taken by the other car owner, just looking around for a car with lots of scratches that they could pin the blame on. If both cars were being driven at the time of impact, then the other driver could have been at fault (although your OH would obviously have been aware of the knock).