I've removed, cleaned and reattached the cranks a few times already, and there wasn't any noticeable improvement. Tried it with the spindles both dry and greased as well!

The BB is sealed and I've already cleaned, greased and reinstalled it once before, and still the creak remained. I'll give it another go at the weekend, making sure the BB is torqued up.

Is it feasible that the crank arm holes have become distorted and that they're the cause?