The LS35/a is warm and well integrated in near field, with a pretty decent reproduction of the human voice. Contrary to the views of audiophools, the later 11 Ohm is actually more accurate than the earlier 15 Ohm (that will be completely out of spec by now anyway).
The P3ESR is far less coloured without any funny peaks or dips, and has rather deeper bass. My main speakers are Quad 2805 electrostats, and the Harbeths are pretty close in neutrality. Unlike the LS3/5a's the Harbeths can handle quite a bit of power, and can be played pretty loudly. Since they lack the mid bass bump, they are also much easier to integrate with subwoofers. I did so very successfully, and the result was massively impressive.
The difference between the little Wharfedales and the Harbeths is quite large, with much greater transparency for the Harbeths. But let us be fair: the Diamonds cost les than 10% of the Harbeths.