I’m not an accountant but have run my own business for 13 years. As the company is in liquidation any claim realised in the companies name would be paid to the liquidator (not the previous owners) as an asset of the company and would form part of the liquidation dividend to be paid out to the creditors.
As the contract is likely to be between the company and the accountant the owners are not party to that contract, so there is no path
There is one (expensive) way they could pursue it in common law as a claim for negligence as individuals (neighbours), but I don’t see that they’ve suffered any loss, in fact they’ve potentially benefitted from the 15% difference between the 5% they’ve been charging & 20% they’ve been receiving.