Underground oil tank feed problem plumbing advice

Underground oil tank feed problem This unit is located in France where they use diesel not kerosene. The feed from the underground tank is not getting through to the boiler. The boiler works fine if I connect the hoses up to a remote tank of diesel. I am assuming that a filter is blocked somewhere but there is nothing between the boiler and the tank. On the top of the tank I can see the return pipe, the supply on/off lever and another pipe which I assume to be the inlet pipe. There is also a plastic bung in the centre which can be removed. I have pictures. My question is whether anyone knows if there might be a strainer or filter within the tank and if anyone has ever removed any of these components to clean them?

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