Ok when alternate picking ive been told to keep my line straight up and down the strings because b4 i would sort of sweep back the lower i went . So my question is when playing all down strokes like the intro to say master of puppets or just strumming in general do i keep my line straight or is it ok to sweep back ??
Cheers Wils.
I find that up and down strokes create the rhythm of your picking. So the down will be the beat playing and the up will be between them. It really helps to keep the rhythm going and keeping to timings.

I also think it's fine to play however your comfortable with though

What part of the intro do you mean by the way?
The Mighty Man Josh!!!
The bit after the 1st palm mute where you play the A,D,G,B strings do i keep my stroke straight down or can you play with a bit of arch like the pic.
That's perfectly fine, as long as you play it clean that way.
I sweep forward quite regularly, Helps with the right hand reaching certain tapped notes.
You could also change this based on tone, closer to the neck the notes get warm and very fluid, perfect for sweeping. Farther back, near the bridge, it's harder (for me) to get it as clean, but the resulting tone color is interesting. Hope this helps