Here's my experience today. As mentioned I was automatically allocated a 'D' (aisle) and couldn't change it online. The check in agent said I could change it at the ticket desk, who said it would cost €24 - a bit different from the €4 on the website, but that I could change on board for €4. I suspect that the latter was just to fob me off.
On board, predictably, I was told it was a full flight and couldn't change. Not surprising.
So, lesson learnt, don't rely on the automatic allocation to give you what you want, pay the £3 in advance.