The most comprehensive list of companion planting is on Wiki. Search on "companion plants" and on "list of companion plants".
Dill repels aphid, spider mites, squash bug (?) and cabbage looper. I think the last two are Americanisms.
The dill needs to be planted in with the cucumber as it also attracts Tomato horn worms so cannot be planted near tomatoes (which - realistically - will be being grown probably in the same tunnel/greenhouse as cucumber). I grow my cucumber in separate bags to the poly beds that the tomatoes are in so that's no problem.
Spider mites thrive in hot dry conditions - such as might be found in a polytunnel in the summer - and puncture the plant cells to feed. Under ideal conditions a single spider mite can spawn a million mites in a month. Squash bugs love cucumber as do cabbage looper caterpillars.