Popped into my dealership and had them look it over. Nothing of concern, keep an eye on wear but suggested just as I'd thought, a stone trapped at some point.
On a side note, I was speaking with the parts guy as i purchased some screen concentrate, he showed me what Lexus is moving over to instead of fluid concentrate. A small tablet, a bit bigger than a Polo, to treat 5ltr down to -5C. Should see us through a normal UK winter. All to save the planet, apparently. More eco friendly, less plastic packaging etc., etc.