If the only 'damage' is the smell of petrol the get the car upholstery shampooed.

I'd agree. My first question was just to ascertain the situation - in case it was someone who was hoping to avoid making a claim, for example, in which case it would just not be worth it.

A potential alternative to changing the seats might (note, MIGHT) be to leave the car parked in a nice hot garage for a couple of weeks with the roof down. That does assume that summer is not over, however !

I don't see that questioning the OP regarding his insurance status is any of our business. There are legitimate reasons for not having insurance. Nor does it pertain to the question put.

Seriously though GE, change the seats. There's plenty of Mini seats available used and it isn't difficult to do. https://youtu.be/jrLkWw2hb7Y

I'm well aware that there are reasons for not having inurance, however, if someone is breaking the law and screwing things up for law-abiding motorists like me, then I make it my business.

And insurance does pertain to the original question as the OP could make an insurance claim for the damage.