I THINK you will find that your contract is with the fitter, on the understanding he supply the worktops and other materials as well as the labour? If he did then then he is resposible.
I would suggest that you get him to invite a Howdens rep to your home to inspect the "faulty" worktops, and with luck, between the three of you resolve this issue.
If a dead lock then perhaps split the cost of replacement three ways.
Maurice, April 2011