Cant Unroot S4 I9505

Hi guys, looking for a little help / advice.
I have a UK Galaxy S4 (I9505) which is rooted and running CM12.01 ROM.
I am trying to Unroot and restore a stock rom via Odin, but it always fails.

<ID:0/004> Added!!
<OSM> Enter CS for MD5..
<OSM> Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/004> Odin engine v(ID:3.1101)..
<ID:0/004> File analysis..
<ID:0/004> SetupConnection..
<ID:0/004> Initialzation..
<ID:0/004> Get PIT for mapping..
<ID:0/004> Firmware update start..
<ID:0/004> SingleDownload.
<ID:0/004> sbl1.mbn
<ID:0/004> NAND Write Start!!
<ID:0/004> FAIL! (Auth)
<ID:0/004>
<ID:0/004> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)

I've tried different versions of the firmware, different versions of Odin. Changed USB cables and ports, but always the same error. Any advice greatly appreciated.

Thanks