Typical - just after I post this I find a fix. 
Found this on Sparkfun - works like a bomb now!
I was using NetHoncho’s library, and was getting only maybe one out of ten readings correct. The rest were sync timeout errors.
I modified his library to retry up to 70 times instead of 35 times, and that did the trick. Look for this line in his lib:
if (retryCount > 35) //(Spec is 50 us, 352 == 70 us)
— and change it to this —
if (retryCount > 70) //(Spec is 50 us, 352 == 70 us)