How weird! Just shows that the two can indeed look similar! Never occurred to me before!
To support the Little grebe camp, to me the bird looks large-headed and small-bodied - a classic combination for small birds. The shortness and thickness of the bill, along with the colour, also rule out Red-necked in my opinion.
RNG has a long, triangular head shape, this has a bumpy look to it.
All scientific stuff, the way I identify birds.