Sorry, the picture is a bit confusing, it was meant to show the type of gas ring in question rather than anything specific. The highlight is unfortunate and ought not be there. The pipe in question is part of the intenals of the ring and cannot be seen without dismantling of the hob. The gas comes from the tap, via an aluminium tube to one side of the wok ring and connected with a compression fitting. The ring casting is 'drilled' to take the gas to the front jet and then continues to the rear jet. The jets in my terminaology are small brass hex fittings with a 1mm hole in them. The bockage was between the front and rear jets.
I assumed it was alluminium oxide, but why would it form?