Fuel injection system with structurally biased relief valve

A fuel pump arranged to deliver fuel at high pressure to a fuel injector, the fuel pump comprising a pressure relief valve arrangement including first and second housing parts within which is defined, at least in part, a high pressure fuel line through which fuel at high pressure is delivered from a pump chamber of the pump to the injector, the first housing part defining a first valve surface and the second housing part defining a second valve surface which is engageable with but separable from the first valve surface, a clamping arrangement for applying a clamping load to the first and second housing parts so as to urge first and second valve surfaces respectively thereof into substantially sealing engagement with one another, wherein the clamping arrangement is arranged to permit separation of the first and second valve surfaces, against the clamping load, in the event that fuel pressure within the high pressure fuel line exceeds a predetermined amount, thereby to permit fuel within the high pressure fuel line to flow to a region of relatively low pressure to relieve fuel pressure within the high pressure fuel line.