From the outside it doesn't look open, but have faith and push open those doors. Inside is, indeed, what I would describe as a tavern - cosy and low-lit with blinds covering the windows giving it the darkened look from outside.
I went at about 5pm on a Sunday afternoon for my first visit and there it was populated but there were still a few tables available. I went back again the other night - a Saturday night - and we again managed to get a seat, but only just.
Discount Suit is an underground cocktail den, whereas this place is more pubby so you won't feel like you're missing the point if you have a wine (as I did on Saturday) or one of the local beers and ciders written on a board at the back of the bar. But, of course, they do still know their cocktails and this makes up the focus of the menu. So on that Sunday evening I had to try a couple in the time I was there.
Bethnal Green is turning into the place to be if it isn't already and The Sun Tavern makes it even more of a worthy destination.