The food you can pick up anywhere at Chester Zoo is usually pretty good. However, the food at the Oakfield is excellent.
It's probably best described as "pub food", but taken to another level. Really tasty, well presented and served quickly too. Even at peak times, the food seemed to come out fast, which is an added benefit if you're trying to see the whole zoo and don't want to 'waste time' in a restaurant.
There was a range of options available, from a burger, fish and chips, to slightly more exotic tacos and similar dishes.
Coupled with a wide range of drinks, you could spend some time in here and completely forget that you were in a major tourist attraction at the same time.
You do need a zoo ticket to visit, so even if you book a table, you'll need to either be a zoo member, or already have a ticket for the day. There's no way around this.
One slight downside was, when asked how a particular dish would be made gluten free, as per the menu options, our server replied "don't know" and made no offer to go find out. Didn't fill me with confidence, so I'm opted for something safer.
All that said - well worth a visit.