OK. Thanks for the help everyone, i'm at a loss with this sort of thing 
I've stopped memtest after 2.5 hours. in any case it appears to just be repeating the tests over and over. It had 0 fails and that was at Auto Volts and 5-5-5-18 timings.
Ive rebooted, changed to 4-4-4-12 @2.2v and rebooted again, and memtest has pulled up 356 errors in under a minute, so i assume its going to fail everything.
Should i RMA this?

I've stopped memtest after 2.5 hours. in any case it appears to just be repeating the tests over and over. It had 0 fails and that was at Auto Volts and 5-5-5-18 timings.
Ive rebooted, changed to 4-4-4-12 @2.2v and rebooted again, and memtest has pulled up 356 errors in under a minute, so i assume its going to fail everything.
Should i RMA this?