Ford Ka on and off battery light?

Ford Ka battery light came on and off a few times over a week. Then is was on all the time but car driving fine, apart from one day when the head lights seemed to dim. Checked the battery, ok fully charged, cleaned it all and the light went off. Now after putting car in for MOT the light has come back on and garage noted flickering battery light on form. Poor old out of work female, any suggestions, can't afford to take to garage