In reply to balmybaldwin:
> will the new battery arrive charged enough to start the car
Probably. Be kind to it those and give it a charge if you can.
> so it's just a matter of swapping the battery out?
The famous "just" - if you have a chance in advance, put a bit of penetrating oil onto the nut/bolt holding the battery strap/clamp in place, and onto the height adjusting bolt (if there is one) on the other side of the strap/clamp, incase the replacement one isn't exactly the same shape/size.
You can always try a "desulfator" on the old battery - some maintenance chargers include this.
Now is a good time to get a maintenance charger and occasionally hook it up to your car if you can; doing occasional short trips to a local shop isn't kind to the battery.
> I realise I may have an issue with alternator
Unless you habitually ignore warning lights on your dashboard there was probably a persistent red battery warning the last time you drove, if this was the case.