How to fix water overflow from AEG Favorit 86040i?

We have taken the side off the machine after the anti-flood alarm kept sounding, each time we tipped the machine to reset it, it would repeat at each wash so clearly there is a problem. (I should mention that on the first ocassion before the anti-flood alarm sounded, we had an alarm which the troubleshooter said meant the syphon needed cleaning, but the manual does not say what/where the syphon is or how to do this!) Since the anti-flood then sounded, we have taken the flat plastic water tank off and cleaned it out as there was black gunk in there, replaced it, but some water still comes out of the overflow pipe half way through each water fill sequence. It's as if the tank is taking in too much water?? AEG Favorit 86040i. Can I fix it and if so what part is needed/action is required? Thanks