New basin waste leaks, only when plug is removed
The holes in the drain are supposed to be &quot;inside&quot; the sink. Most china bathroom sinks have a double bottom in the region of the drain. The holes in the drain for the overflow must be inside the cavity between the two bottoms. The holes in the drain cannot be outside this cavity or water will pour out when the sink is draining.
Some bathroom lavatory sinks are single bottom and have an exposed overflow tube conducting overflow to the drain. These may require a different kind of drain. You may have a mismatch between the sink and the drain.
EDIT
You cannot (permanently) stop a drain leak from the outside using plumbers' mait (putty) . There is something mismatched and the leak can be expected to reappear. Consult someone in the UK. They will quickly recognize the problem.
Perhaps the seal you are using on the bottom (outside) is not the proper one for this application. I think this seal is supposed to be conical on the end to protrude into the cavity. This causes it to tighten around the drain when it is compressed.  Or the holes in the drain are too far down to be fully enclosed by the cavity around the drain. Perhaps there is more than one type of drain and you are fitting the wrong one.