Running

Why does cycling cause tight glutes / inflexible hips?

My left glute is continually knotted and regardless of how much stretching out does come good.

Starting to give me hamstring problems too now, really slowing my run progression.

Are you stretching regularly? If you haven't already i'd suggest investing in a foam roller. I try to do this routine everyday: Joe DeFranco's "Limber 11" (flexibility routine) - YouTube
&v=FSSDLDhbacc It has helped me both with my running and weightlifting.