If batteries aren't mentioned then get a couple of spares. Plus if you get a speedlight, get some more batteries
Scout out the area, to see if there is a nice place for photos, or at least a pleasant background. When I got married we were fortunate to have a really nice garden and park behind the registery office and all the photos were taken there.
If its registery office then all liklihood you will only be able to get the bride entering, a shot from the back of the room, a mock up of the signing of the register. It is likely that you won't be able to take during the service except from the back without flash.
Make sure you get some of the groom and usher/best man before the bride arrives and perhaps some of the guests.
After the service you will obviously have a bit more time to shoot the guests informally.
Do the largest group first and make your way down to the B&G. If you can, tell the guests to goto the pub and take the B&G away for some private photo time.
Bring sweets to bribe the kiddies. Bring a white brolly or two for the bride in case it rains - also good if strong sunlight.
Make the guests laugh, have fun yourself and the photos will improve no end!
Sorry if I have repeated stuff, but best of luck
