No 11 help needed

Hello dont known this is the right section

I recently purchased this Optimus no 11

but its leaking from the valve underneath the burner
(look at the third picture)
How do i remove it what size spanner is it (4mm or 5?)
What kind and size of gasket do i need?
Thanks in advance

 

The jet? If so, articulating tool or small spanner that should have come with the stove.
Duane

 

I would think 4mm...

 

Thank you

i tried to start it yesterday to check its function

had to tighten the spindle slightly
It now works perfectly