Phillip_James:
I was under the impression the Due had internal 3.3v pull up resistors on the I2C lines. Is this correct?
Yes, this is correct. However, the DUE is a 3.3V system, and the SRF is most-likely a 5V gadget. If that is the case, then you need a level translator, as the logic thresholds probably do not agree.
Nevertheless it is working as is, but just not 100% reliable
That is the point.
-Chris