Delivered today was the cheapest (GBP 2.49) Micro USB OTG adapter I could find on Ebay UK.
Listed as: LG Optimus G2 D802 E960 Google Nexus 4 OTG Adapter Host Cable USB to Micro USB
Using the OTG adapter with the Fiio E18 works in both PC and Phone modes with Power Amp, Amazon MP3, Google Music, Tune In RadioPro and the LG Music player all working perfectly.
Incoming calls play the ringtone over USB audio but on accepting the call audio goes to handset only.
In addition my old Hotaudio Bitperfect DAC also works fine as well.