Fuel Feed Hose Internal Diameter

I have brought a fuel heater for my 2000 TD5 due to running it on Biodiesel which I have for the last few thousand miles. I want to intstall this just before the injector manifold, however, the connection from the hose to the manifold is of a special type and I can't determine the fuel hose diameter. Before I go ahead and cut into the hose, can anyone tell me the internal diameter of it so I can get some tail ends for my Biodiesel heater?

Thanks.