The original black plastic side panels on my DT400 twin shock are quite scratched. 
As these are like hens teeth and I don’t want to stuff this up, I’m seeking advice from anyone who has successfully ‘polished out’ the scratches on theirs.
What’s the best product (uk) / best method please? 
Thanks..

I have an two sets, one in ok shape the other is slightly less desirable.  But having said that I'd look at the link below, KDI sells them new, which I'll be buying a set for my DT400C restoration...

Side covers

To stay with originals is what I typically do. KDI has great products and very true to original look (From what I have bought). I have not bought their side covers. The plastics can be polished like any other surface you start by sanding the surface to get rid of the scratches and imperfections. Then continually go to finer and finer sand paper at the end I use 3000 grit but there is also 5000 grit. Worn down 3000 leaves an almost shiny surface. Finish up with polish of your choice. Meguirer’s has various grits and you want to end up with a very light cut and then their pure polish. This is not for the faint of heart and all the products add up in dollars. You may just have to experiment but I’ve cleaned up several sets of those side covers along with the plastic fenders and covers for the later bikes.