Oil Filler cap leeking

I had my engine steam cleaned and noticed that oil was leeking from under the oil filler cap, when you google for a new seal for the oil filller cap, this comes up as the result, now, When i ordered it I knew it was prob not the right bit, but for £3 I would give it ago.

Its far to small and to tall to be gasket for the filller cap, so just what does it do?

Also any one got a part number for right part?

Thanks

its the right one for 2.5 diesel it goes on bottom of cap which pushes into rocker box