AEG F86050VI -PNC 91123472602

Do you ever get those weeks from hell? Where everything seems to be against you, and nothing makes sense? Such has been my week :-).
I went to this dishwasher and found that the dosing tank was blocked up, and the base was filling with water. Simple I thought. Order a new tank. So I ordered part no 4071349759 as listed on the Zanussi website, through Maddocks. I duly went back and fitted it and expected it to work perfectly, just like every other one I have ever done.But no. As soon as it began to fill, there was a leak from behind it, from a small plastic flap that is covering a couple of holes, then the overflow started to fill up the base again. I took the tank back off twice to double check I had not done owt daft, but all is as it should be. At this point, with brain ache and a complete lack of belief in the laws of physics, I gave up. The flow to the sump is clear, and as there is basically just one way in, and one way out for the water to flow, (apart from the water softener unit), I have come to the conclusion that the tank must be faulty. The problem with this is that although parts can be faulty on arrival, it is rare. But then this part is pressed and sealed at the seams, and I imagine could quite easily be assembled incorrectly. Any ideas please? Any assistance would be much appreciated. Cheers. Steve.