That's how I assessed which side is input and which output. Looking around the Ford Forums I couldn't find any indication of a structural problem with the filter being the wrong way round. It seems that the flow is more restricted and so it cloggs more easily. As it's done more than 40 miles the wrong way round and the tank was almost empty, rather than releasing that tiny bit of crud that is bound to be in there and makes all the difference into the injectors, I think I will leave it as it is for now and invest another 10 quid in a new one I can put in the right way round at some time.
