I've recently fitted a kitchen. The furniture was budget but decent, and
put together by a cretin at a local Travis Perkins shop.

I use "cretin" because he gave me a mismatched bath waste and trap,
meaning that the trap wouldn't screw tightly to the waste. So I had to
get another, which leaked, and then another, and now have a snug
water-tight fit.

And I think he's done something similar when deciding on what basin I
need - I think he's given me the wrong waste for the sink. THe sink is
one of those with an integrated overflow (ie which is built into the
porcelain).

He's given me a waste which appears to be the right one, but when it's
tightened up, it leaks. Water appears to be coming from the union of the
sink and the waste underneath, where the plastic bolt is which holds it
in place. Before you ask, yes, I've put rubber washers on both sides,
and after discovering the leak, I took it apart and put some silicon in
to help too. No luck - it's still leaking.

Any ideas? I'm tempted to get a new waste and just start afresh, but I'm
fed-up of wasting money.