Assume you have covered the usual checks in the instruction manual. I found the following on the BOSCH site when trying to solve draining problems: Frequently Asked Questions

There is also a video (How do I unblock my dishwasher pump?)

This didn't help me, so I ended up disconnecting the dishwasher and lying it on its back. I removed the two (brass) screws on either side of the baseplate at the front, then needed to remove the lower 6 screws inside the door to remove the front (white) panel of the door. This gives you enough room to remove the next metal plate to let you into the area in the base of the dishwasher. You'll see a short (30cm or so) length of pipe which leads from the drain pump. The pipe is just a push fit, so I took this out and gave it a good clean.

This has improved my problem (dirty dishes and dirty water in the DW at the end of the cycle) but it is still not 100%. I think I might need to investiagate the drain further down the line for more fouling/blockage.

I'm a DIYer and have never taken a DW apart before!