Now as an update, the DHT11 has burnt out! I was starting to get fewer and fewer readings, getting a checksum and then timeout error from the arduino code I was using. Eventually I never got any returned values, so decided to take a look. After checking all the cable runs, etc, I then added the pull-up resistor that was recommended in some locations. But unfortunately still nothing. So I decided to prise open the little blue plastic package, and the problem was quite clear:
My best guess is a large amount of moisture gathered, condensed and the water droplet caused the short to take place and kill the sensor.
So I've now got a replacement DHT11 and trying to work out how to safeguard the next one against damage and how to better sense the moisture in the dryer without destroying the sensor too!