Hot temperatures can bring out the licorice scent, which is similar to fennel.

Thanks for that! I'm so relieved that it's not caused by another plant.

Changes from other plants would occur in the offspring, not the current generation.

And it couldn't occur between basil and fennel anyway as they can't cross. They are not in the same Genus, or even the same family.

I didn't think it was due to hybridization as that wouldn't be possible. I've read that some plants can be altered slightly by their surroundings and I wasn't sure if this was the case with basil.