As stated in post #6 onwards I am using the BPI Debian/Jessie 15.08 release which uses u-boot so fex files are used for pin config not DTS ... We just cant afford to spend any more time trying to get uart3 working right now ...
However since my last post we have had very good success using uart7 (GPIO pins 36 & 38) as an alternative which has allowed us to progress with testing BPI M1+ ..... unfortunately it also meant we have had to create a prototype BPI M1+ specific version of our daughter board with a 40 pin connector which will not be backwards compatible with Rasp Pi which was our original intention.
However having seen the performance gains between BPI M1+ against Rasp Pi Model B this may not be so much of an issue now
For now please consider this thread closed, but we would always be interested if anyone is eventually successful in getting uart3 working in the future ...
Cheers
Jay