A4 2.0 petrol oil leak left rear of the head

[Cpt.cliff]

image.jpgimage.jpgimage.jpgHi guys,I'm new to the site so go easy, I was on my way home when I had smoke coming from the engine bay, stopped and could see oil was coming from a hole at the back of the head all over the cat and burning off. Got the car home and had a closer look, it's a round hole to the left of the temp sensor and is 26 mm in diameter. Ordered a part from audi that's a 22 mm core / freeze plug so won't fit my 26 mm hole. The question is do I need a 26 mm plug or is there something else that go's in the hole that's fallen out. Thanks. Just added some photos the holes uk drivers side. Thanks again