crwdns2893792:0crwdne2893792:0 Reece ,
I had one with this issue a while back, a 13" 2015 model. IIRC, it would only show the diagonal stripes when in an OS, or intermittently (holding option key on boot would show picture fine I think). The issue was related to the logic board, I believe it was related in some way to the CPU integrated graphics. I didn't spend too much time on it though, since there was no physical or liquid damage present to suggest a cause, but it was definitely logic board related. Not a very common issue, but seems to be a rare fault on these and have seen 1 or 2 A1278 models with the same problem. Try outputting video to an external screen to be sure and if you are still having the same issue, try a replacement logic board.