Toyota Aygo 1.0 2007 - One of the spark plugs is bigger?

[Auxouis]

Toyota Aygo 1.0 2007 with 3 cylinders.

I was checking the spark plugs. I removed the 1st and 2nd spark plugs with a 16mm socket and checked. The same 16mm socket doesn't fit on the 3rd spark plug. I assume the previous owner managed to fit the wrong size socket? Or a socket with a larger nut?

The next size up that I have is 21mm and it's obviously too big. I assume it can either be a 17mm or 18mm. Or could it be an imperial size? 17mm seems rare and obsure, I've been looking for one on eBay.

[flash22]

What make are the plugs ? yes plugs come in different sizes - have a look down the hole and see what you have take a pic if you can

you may require a thin wall spark plug socket, 14mm or 18mm