Re-creating the radius in the rear arch

I'm working on the body work preparation on my MK1 and it's had a new rear arch fitted before I bought it. It looks like it has been welded in around the curve where it blends onto the rear panel around the top of the arch.

any tips on how to re-create the small radius around the top of the arch please?

Oh that's a pain! If I understand you right, lead loading could be an option? It should be a more robust method than a load of filler.

However you do it, shaping it may be a pain in the back side!