I know this is horrible code btw (inluding unused variables etc), but getting to this solution took me so long I didn't have the patience to stick around and refactor!

I like this solution, not showy, but clean and concise. Good job.

This is great. I didn't know about URI.parse or that #last could take an argument! Thanks

I'm inclined to agree. It's not testing your coding ability so much as your knowledge of powers and big numbers. That said, once you've found the 'rules' to determine a last digit, it's more straightforward, but then - as you say - you may as well have just looked up the solution.