Here is an interim update.
With the updates that Graham has already mentioned earlier in this thread I seem now to be able to run WizNet (UDP and TCP/IP) together with SdFat and UTFT_GHL and they all coexist on the SPI bus. I'm not completely sure what I've done to get to this happy state, nor am I fully convinced that it is definitely 100% working. I'm going to need to spend some more time on this over the next day or so but I will write up whatever conclusions I reach, as soon as possible, here.
Regards,
John.