Mentzer red box D&D from Trapnell's model shop, I would guess 1987 or 1988. Age 10 or 11. I was already into Fighting Fantasy books and White Dwarf and I already had some GW figures.
What was noteworthy was that when I tried to buy it, a man (fellow customer) in the shop said no you don't want that, here buy Runequest instead. But I was not swayed.