Miele G646SC PLUS inlet/drain warning light issue?

I’ve had the dishwasher for about 7 years. For the last 4 years or so the inlet/drain light has been coming on. The drain outlet, sump, filter, impeller etc are all okay. Water has been gathering in the base tray which initially I could drain by tipping the machine and it would then work again. This doesn’t work anymore as the tray is filling up with the ‘water protection float’ and microswitch system triggering the drain pump to run continuously with each use. I’ve tipped the machine onto the side and have taken off the bottom tray. I’ve cleaned the float which was slightly ‘gunky’ but this hasn’t made any difference. I couldn’t see any trace of where water may have been leaking from. The main seal in the centre of the dishware base appears to be okay. I’m guessing that there is a leak which has got steadily worse over the years. The only thing that does look slightly worn is the seal around the ‘door frame’ at the bottom. Has anyone any ideas?
Also what is the black tube which is open which ends just above the tray and looks like it goes to the ‘salt unit’?
Many thanks

Graham
January 2010