In the end I bought some Seagate Exos 7E8 4TB SAS disks which were immediately recognised by the H750 controller as non-RAID disks and they work fine in the R540 I installed them into although the controller utility doesn't display the SMART info for these (which isn't mission-critical). So problem solved - I'm ordering another twenty of these. I've no idea why the WD disks caused so much grief although they do seem to work fine in other Dell systems that use different RAID/HBA controllers so the expenditure hasn't been wasted. I chose these Western Digital disks because they are actually HGST disks, WD having bought up HGST (formerly Hitachi, formerly IBM Storage) since I've found HGST disks to be very reliable over the past decade.    
I have bought four new R540 servers each fitted with the H750 RAID controller with the intention of adding five 4 TB SAS disks to each and using these as independent drives rather than as a RAID array. If I create five RAID 0 arrays each with one disk in it, will this work?  This certainly works with the older PERC 5 and PERC 6 controllers. Andy
Using individual disks in non-RAID mode on a H750 controller