I am trying to indentify a part of my fuel line?

I am trying to indentify a part of my fuel line which is some kind of valve/themostat type device.
It has cyclindrical shape with a sticker of temeratures on the side and a small black button on the otherside, like a primer button. Anyone in the trade have an idea??
It is situated by the external wall before pipe enters wall. I have a combi boiler which uses Kerosene

Thanks

James_M
January 2011