I'm pleased enough with the result (and how surprisingly easy it was to achieve), although it isn't as good in real life as it appears in these photos; the 'stipple' isn't as pronounced on the repair and it means that the spray paint finish is shinier on there than the surrounding areas especially on the top. Replacing the label in its original position makes it go over this 'boundary' betwixt repair and original and goes a long way to disguising the difference - hence it's going back on!
It's a monster of an amp and I expect I'll be keeping it long term
. Many thanks for all your help.