The workers are sub-contractors.

I would have looked into this before make any major purchase.

It is too late to say to you that buy the kitchen and find a local tradesman is a far safer salution.
Saying this, I got stune once. A family friend recommended a builder he knew. Then the builder rocommended his dad, who turned out to be a cowboy. The dad plastered our bathroom ceiling. After he left, I counted that there were at least 100 holes. Called him back and he turned up with a pot of Polyfiller...
Because we are nice people, we paid him on the day; so we could not get our money back.
We paid out another ?250 for another plasterer did it again.

Hiring people is a risky business.
Make sure you hold some of the money back, so if there is a problem that person will come back fix it.

Homebase, the only thing you can do is being persistant.
I would take some pics of the shoddy job and go back to the shop where you paid, ask to see the store manager, show him the photos and ask him what he's going to do about it.