Trying to apply OTA - Recovery error "No Command" topic

I've been out of circulation of the whole xda thing for a while with my HTC One M7 (AT&T version) but I decided to convert the thing to GPE and get updated to android 5.1.

The conversion went smoothly using this guide but now I am having some troubles applying the OTA to update to 5.1

I have tried unrooting by using the unroot option on SuperSU and then flash the OTA using stock recovery, only to find that when I want to access stock recovery I get a screen with the android alien lying down and the error "No command".

I figured I could flash the stock recovery with fastboot to fix this (downloaded 5.11.1700.3 from here) but I still get the same screen and error when selecting the recovery option from the fastboot screen.
In a desperate move I also tried flashing CWM and trying to flash the zip file but the OTA fails to install that way.

What am I doing wrong here?

Thanks in advance for your help