You can get a free battery check at any Kwik Fit Barry and you don't need to go to a main dealer for a new one. Any decent car accessory place will do them. Make sure, when they give you the battery, that the terminals are the rtight way around.
Have you checked the cells to make sure they are all full? Top them up with some distilled water.
Try charging the battery up and see if its any better as soon as you take it off of the charger. If it is but the next day its sluggish again then its a good bet the battery is knackered.