In common law, on "grounds of public policy" contracts to defraud the revenue are illegal contracts.
So - looking at this again:
Op pays "cash" for a service (blocklaying) - Op knows/suspects that blocklayer won't pay income tax/vat. Op's not happy with the quality of the blocklaying - can Op sue - yes.
Op takes the risk that the blocklayer will claim in his/her defence that the contract is illegal as both parties were in effect defrauding the revenue and he would lose.
But the blocklayer takes a far bigger risk using that defence, I think, though - as he would in effect be admitting in court to defrauding the revenue and face a revenue audit/penalties. So he may have no other defence, and the op would win.....
So - looking at this again:
Op pays "cash" for a service (blocklaying) - Op knows/suspects that blocklayer won't pay income tax/vat. Op's not happy with the quality of the blocklaying - can Op sue - yes.
Op takes the risk that the blocklayer will claim in his/her defence that the contract is illegal as both parties were in effect defrauding the revenue and he would lose.
But the blocklayer takes a far bigger risk using that defence, I think, though - as he would in effect be admitting in court to defrauding the revenue and face a revenue audit/penalties. So he may have no other defence, and the op would win.....