If the seller's advert states the mileage, (or doesn't state that it isn't accurate), then he is liable.
Just because he didn't change it himself, doesn't mean he should benefit from misrepresenting the car.
I would speak to Trading Standards & get their view.
Last year, I bought a diesel Jeep with what was described as an 'exhaust gasket needing replacing'. The owner told me that this was the diagnosis of a garage.
When I got the car looked at to get the work done, I was told it was more likely to be the head gasket. Sure enough, got a second opinion and it was the HG.
I contacted the owner, who denied all knowledge. He claimed that as he wasn't a mechanic he didn't know.
I contacted the garage that told him it was the exhaust gasket. They told me that they hadn't looked at the car, just told the owner that it could
So, with that information I went back to the owner who said it was tough, sold as seen. He was not a mechanic, blah, blah, blah.
Next step, Trading Standards Consumer Direct. They said private sale, so my responsibility to check the facts.
I paid by credit card so I asked them to step in, which they did and withheld payment to the owner until it was sorted out.
Nothing they could do, it was a private sale, so I'm back to the old owner.
The cost to fix was £600 or £300 if I got a mobile mechanic to do it. I went to the owner and asked him to pay and eventually got £100 from him to go away.
Lesson is, if you buy private, you are fooked if it goes wrong despite what the law appears to say.