I just won a 63 2.8 nikkor and 60 rodagon on ebay and have recently used both for 20" prints. The problem is completely solved with this slightly longer FL and I now have perfect corners that are indistinguishable from the centre. Whether it was some sort of mechanical issue, or lens curvature of field I will never know, but these longer lenses have cured the issue 100%. And the pair were far cheaper than a APO anything. Whatever the reason, that 10mm leap in FL has made the problem go away
Both perform stunningly well and do all i could ask so no need for any more lenses.