I have an annoying cluster of bubbles in the middle of a 12inch diameter resin lake that I could remove with a plunge router, maybe 15-20 mm deep, 50mm diameter. Could I top up with more gc50 and no one would ever know?

As long as you are careful to keep the hole clean and give it a good sand for adhesion then you should be able to blend the repair in. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant