This has now been resolved with this assistance from Dell Support. There was a setting under System Setup > iDRAC Settings > Thermal > Fan Speed Offset that had somehow been set to High Fan Speed Offset. I set User option back to Default, and the fans all immediately reduced speed and quietened down.  
I have just performed a clean installation of Windows Server 2012 R2 on a new RAID 1 pair of 300GB SAS 15K.  I tried both methods, directly off the OS DVD and using SBUU. BUT the fans will not slow down after power on. Once the memory check is done, and it starts to 'configure iDRAC' the fans spin up to around 12,000 RPM and DO NOT slow down. I have updated the BIOS to 2.4.3, the iDRAC7 to 1.66.65 and the Lifecycle Controller 2 to 1.4.2.12, all with no change. I have also tried changing the 'System Profile Settings' in the BIOS, with no difference. The previous Windows Server 2012 R2 installation was working OK, but just installed on 1 * 2TB SATA HDD PLEASE can anyone help me?
Dell PER720 very high fan speed!