The gas light heads in Stead Lane were 4 sided with a taper and were about 14" high. They were not ornate, just functional. The mantle was about twice the size of a Tilley lamp mantle. None of the ones you posted are like the Stead Lane type. The cast iron poles are similar though I'm fairly sure the bottom was square. I'll try a google for gas lights or lamps and see if I can find a similar one.
The only sewer gas light I know of is on the footbridge over the river from Castle Square at Morpeth. I was told it only ran off the methane from the sewer pipes and no coal gas and later natural gas was involved. Eggy, your photo shows a sewer gas light using piped gas as well as methane. This may be the case at Morpeth because should there be a shortage of methane the light would go out. The Morpeth one burned 24hrs a day as you would expect to burn off the sewer gases. I'm sure the original light is still there but don't know if it is converted to electricity.
Jammy