Battery bother

[Norretal]

I got a call while at work today from the wife saying her car won't start (A4 3.0 TDi) The car has been fine with no early warning signs of battery trouble and she only just used it yesterday, as well as the previous couple of days. I went home and took her to work and had a quick check around and there appears to be no interior lights left on etc, so I'm suspecting the battery is dying. I'll put it on a trickle charge when I get in tonight but if that doesn't work are there any common faults or suggestions anyone can make? If I do end up purchasing a replacement is there anything I should be aware of before disconnectiong the old one? I appreciate this isn't a car that many on here are experienced with but I'm hoping the common traits apply through the whole Audi range and I can save myself some trouble if forewarned

Thanks in advance