Well I don't know what you've ordered, but I've never had a bad meal there. The dry meat, chana/aloo gosht and tandoori lamb chops are highlights IMO (the pumpkin curry is also good as a veggie dish and the Lassis are top notch).
Like Owen says, Lahore is a banging curry too, but I reckon Tayyabs just edges it.
Maybe it's not to your taste (if it is the same place and not a takeaway freeloading off the name - they don't deliver as far as I know), but a restaurant doesn't stay so popular serving crap food. If you're used to currys tasting a certain way that might be the reason. A whitechapel curry tastes very different to a bradford curry, for example, depending on where most of the 1st gen emigrated from. I love em both.
Just had a Sushi takeaway in the park myself - luverley.