Even if you aren't staying there, the best option in town is the HongKy Hotel and Cafe Restaurant. The menu is limited but has a few breakfast-type egg dishes, a selection of noodle and rice-based meals (the beef noodles are plentiful and very tasty) and a sufficient range of hot and cold drinks. On our last visit the courtyard area outside the guesthouse wasn't set up for visitors but we're sure if you asked nicely they'd put out a few chairs, alternatively their cafe on the main road, just by the entrance to the alley, is a pleasant enough spot.
For a quick bite, there's plenty of com and pho, or head along the road towards Dien Bien Phu for a couple of uninspiring but more comfortable venues, both with limited English menus: Hoang Quat asks you to "Come in please" and announces "Safety, Clean Food for Health for Better Life" — and who doesn't want that? — but it's a rather gloomy looking place with plastic chairs and frilly tablecloths and is apparently only open between 19:00 and 20:30.
Over the road, Thanh Thuy is a large wooden venue prominently displaying a Pho and Com sign but with a wider range of local food available inside. They were a bit cagey about opening hours but confirmed they were open in the evening.
Hoang Quat: 73 Truong Xuan (Highway 6), Tuan Giao. T: (0230) 386 2482 F: (0230) 386 2060
Thanh Thuy: 74 Truong Xuan (Highway 6), Tuan Giao. T:(0230) 386 2480