Issue with Car Clock Dashboard

I have an issue with my Ford Fiesta Finesse 04 Reg.
Lately, when starting up the dashboard has been a but funny, the petrol gauge flashes up and down as if loading - this never used to do this?
Also, the time on my car clock is losing time, everyday but about 3-5 minutes.

Also, not sure if the battery just needs to be replaced - and it may fix all the problems but when I got the car I used to be able to leave the radio on and the fans without the engine running but now I can't why is this?

Should I replace the battery and see if this fixes all issues, or is the dashboard a seperate thing altogether?