I have V reg Marea 1.6 Weekend but no manual. A warning light has appeared on the dash today. Looks like a gun barrel with dashed lines being fired from it. I am thinking it's the fuel injection system? Am I right? and can it be solved be adding a cleaner to the fuel tank?

it is the fuel injection system fault light, you should think of it as the engine management light (compared to the average car). when it illuminates it can be due to many possible faults. if the car is still running ok it usually means a sensor has gone out of range but is still working. a diagnostic check will usually identify the problem. using redex or similar can often put the light out temporarily, if that happens it suggests the lambda sensor is at fault.