Ah - today's session was better - 10k in 45:15 (PB for me is 44:02 - so nothing earth shattering).
I'd been forgetting to concentrate on the transition from the leg-drive in the first part of the stroke, to that last upper-body tug at the end of the stroke, when you're trying to row harder. When it feels like you are accelerating through the whole length of the stroke, it feels like you're doing more - and the split time certainly shows it.
I rowed a kind of saw-tooth in effort; 1000m of 2:18, followed by 500m of 2:12 and finally 500m of 2:05 or less before easing back off to 2:18. Varying the pace like this, and concentrating on maintaining a steady split, certainly makes the session pass by faster.