Ok so I was attacked by dawnguard whilst leaving the castle, serana turned one into a zombie and it then turned to an ash pile. Now I left the area went all the way to riften waited 11 plus days and the not only is the ash pile still there, but all the bodies and weapons they dropped are still just there on the ground.
Has anyone else had a similar problem?

Hm can't say i've had this problem, but i know i had a simular problem in whiterun when it was attacked by a master vampire and thralls. Just Ash pile left over never seemed to disappear,i couldent even loot it. It just remained there. xD as for weps on ground i think i just got to point where i just picked them up and sold them.

I had the ash pile glitch before, but the patch they released fixed it. I can still loot this ash pile and interact with it, it just won't clean up. The weapons not cleaning up isn't a problem because as you said I can simply pick them up and sell them. However if I keep getting attacked outside the castle eventually the bodies and ash piles are going to pile up. Also interestingly enough I think this problem is exclusive the castle volikar area as I have revisited areas where an attack has happened and it cleared up as normal