Thanks @techknight. Yes the online resources tell me 0000000F is software related, which confused me. But maybe NMI is being triggered somewhere, causing this error code to appear. I will check the NMI area carefully (which I haven't done yet apart from adding R11).
At this stage, I am assuming POST is successful (the fact initial boot chime sounds followed by the screen coming on and going to memory check) so it's either memory or NMI as you suggested.