I have the exact same problem at the moment, speedo not reading below 20, then eventually after a slow crawl in traffic, stops working altogether. If I plug in a diagnostic handheld tool, it doesn't read the codes, but it does appear to reset whatever is stopping the speedo from working.
Garage replaced the Rear right speed sensor with no success. They had to dismantle the hub to get the sensor out as it was stuck, so i imagine they checked the ABS ring for damage.
Are you any further forward with your problem?