***FIXED!!!***
Thanks to all that helped along the way, Thank you @subway_lunch for all of your help, sorry I couldn't get your method to work!
Just to clarify to any others trying to do this fix, I used the code above and made it into a VBS script. then added that script into the logoff policy in GPM and then linked that to the relevant users. for the first time to see If this worked I had to Log on, log off and then log on again to see the updated wallpaper
Having said all that I would be interested in this method for future development, I've heard about this but not really looked into it before, is there a help sheet or guide that you recommend to get started with learning about this?
Thanks again all.
Delleropk