one of the goldfish in our garden pond looks like it has swallowed a ball!?

It could be a female full of eggs...it could also have dropsy. Dropsy is untreatable, the only thing you can really do for them is put them out of their misery, or keep the place clean and hope.

Here's an example of a goldfish with dropsy, they get HUGE before they pinecone: http://www.wetwebmedia.com/CypriniiformPIX/Goldfis...

The pineconing, final stage and should that occur if it IS dropsy(a photo from you would be better, it could be fat, bloated, have a tumor, carrying eggs or sick), it needs to be put out of its misery. http://25.media.tumblr.com/f162a72923e4f48ce86bd77...

This is a photo of a goldfish with impacted eggs: http://www.goldfish-emergency.com/forum/attachment...

*ADD* If it looks like the first two photos at all, it's in misery. lol Dropsy and Eggs aren't mix-upable, really. If you can't tell, killing it when it doesn't seem to be suffering is probably a bad idea, I cannot see the fish to tell you! =P

If she's just very plump, especially toward the rear, she may be carrying eggs. If it's sticking out to one side, could be a tumor, could be impacted eggs.

Do you know how to tell males from females? For one, if it's male and looks that way, it's not good. Males have these sorta...little spots on their gills and around their pectoral fins, females don't.

It could be pregnant..I'm not saying that it is but if it is really fat it could be. I have 2 pregnant fish at the moment and they look huge!

Its either pregnant or fat.

it pregnant. congrats