Aha! Killed them! It seems that rockets are mostly a waste of time on the Hectors, as rifle fire can actually stop them charging their lasers. The upshot of that was me getting past that level.
I had a level full of red ants at the beach then, which was easy. I actually completed the level three times as the armour drops were plentiful and I made use of them. Then, more ant/red ant/spiders in the city with nests to blow up, and finally back into the giant caves full of insects.