Rover ticking when trying to start?

Hi, I started up my car today and the battery light on my dash stayed on. I was only making a short trip so I decided to leave the car running for 15 mins to give the battery a chance to charge. I drove back home and the abs, srs and the engine light came on a which point my headlights started to dim. Eventually my speedo and rev counter stopped and my headlight finally gave out.

I tested my battery and it was still over 20% charged and all the internal lights work. When I try and turn the car over there is a clicking sound.

Car: Rover 25, 2004