It depends how cheap you are looking for. Capital Kebab on The Cut
is probably the best place for cheap. It's nice food and they tend to give you lots of it (you can sit down in the back).
has lots of restaurants, for various budgets, so they should probably just go have a look. Ev on Isabella Street
is always nice, the bar does quite a cheap menu (it's marked on that rather good map you were sent on the other post as Thai Silk, between Joan Street