how can i fix my ford fiesta ?

hi, i have a M reg Ford Fiesta si, about 2 weeks ago i was driving on the M25 and the battery light came on, i hit a bump and it went off. Then for the rest of the journey it came on and off intermittantly. The light was mainly dim but occasionally falshed to bright. My first thoughts is that its a loose connection. This has now been happening for 2 weeks and tonight the power faded anf died. The only way to start it was to jump start it. I think it might be the alternator but not sure ,can anybody help?
Many thanks Gregg