I know nothing about cars. I have a 57 plate audi a3 1.9 diesel SE and when turning the key to start the car, it sounds very sluggish, sometimes it doesnt start first time but will do after 2 or 3 attempts. i think this is an old battery which need replacing???

i have inquired about getting a new battery with audi and will go ahead with it, but i am concerned of my self diagnosis since i know nothing about cars. Anyone got any ideas what might be causing this? It has been regularly serviced, last time in December, we also had a new timing belt done then.

Thanks in advance