Back to radiators ..... I had a local machine shop look at the brass plate, but access was too restricted to do anything so I used a selection of your ideas above ....
There was about 0.5mm between high and low points, so I filed down the worst with a diamond chisel sharpener, then emery cloth wrapped around a flat metal block and then finally a thin skim of chemical metal , sanded
I refitted the pump with 2 thin gaskets and no "goo" et voila, no leaks after several hours .... so the rad. can be touched up and fitted
As an aside, I took the bonnet catches apart again, fettled a couple of the screws that were fouling full movement of the clasp, greased everything and fitted split pins
All working nicely