Puzzle ZIMH 

Workings

Hint
It doesn't matter how far it was to Manchester.

Answer
Blake.

Reasoning
If we call the total distance T, we know that:

   on the way there Alex drove 90 miles, and Blake drove T - 90 miles.

   on the way back Alex drove T - 100 miles, and Blake drove 100 miles.

So, in total:

   Alex drove 90 + (T - 100) = T - 10.

   Blake drove (T - 90) + 100 = T + 10.

So Blake drove 20 miles more than Alex.