To summarise what we did to reach this happy state of affairs :-
SdFat with UTFT_GHL works fine as relased, the DMA code used in UTFT_GHL is the same as that used in SdFat, so there is no real surprise this combination works without issue.
UTFT_GHL and Ethernet, not happy, ethernet hangs after access to the Font IC.
SdFat and Ethernet, not happy, ethernet hangs after access to the SD card.
The solution here is to configure SdFat, in SdFatConfig.h change Line 81 to #define SD_SPI_CONFIGURATION 1.
This should fix your problems with Ethernet and SdFat.
To effect the same fix to UTFT_GHL,, in HW_ARM.h (currently line 39) change to #define USE_SAM3X_DMAC 0.
That's the end of the good news unfortunately, the result of these changes is significantly slower SPI performance.
I have an Ethernet R2 card now (THANKS, you know who you are), so this is not the end by any means, if we come up with another solution, it will be posted either in this thread or a new one.
Hope this will save people LOTS of time!!