I clearly remember stopping here on the way to our family holidays on the Lleyn Peninsula as a child. We lived just outside Birmingham and went there every year for a holiday, and stopped in this place several times. I'm guessing it was somewhere along the A470 as that was the route we always drove, but perhaps we turned off to get there. Fairly certain it wasn't Coed Y Brenin, as it was much quieter than that, no facilities etc. I think it was between Machynlleth and Porthmadog though.
There was just a car park in a pine forest, with a low stone wall round it. I remember there being pine needles on the ground. And then we went for a short walk through the pine trees to a small river that ran through the woods. My memory is very clear of the reddish brown floor of the pine woods giving way to this rocky river.
My parents claim to have no memory of this place. So I'm asking MN instead 🤣
Last year we ruled out Tyn Y Groes nr Ganllwyd and Aberglaslyn. I'm off to the same area again in the morning and would love to try again to find it!