Ok so I connected the 816 up to my rme HDSP and same issue,regardless of which clocking method I use, the 816 as a slave will not follow the RME master clock rate.
The only way it will work is by having it connected via FW and manually adjusting the sample rate,surely this can’t be right.
I’ve wasted a whole day on this so far 
Now going to see if I can find the SPDIF workaround…
MC