New Look at Crown Point has (or at least had in 2011, that was the last time I bought maternity stuff) a good selection and is close to a big Asda Home Living that has maternity stuff too. Next is there too, and Mothercare. Peacocks used to be opposite, but sadly no more.
I found that less and less shops stocked maternity wear in store. Dorothy Perkins in the white rose used to have a decent range although last time I looked was down to just a couple of rails (before Christmas). I found next online to be the best place, wide range of nice stuff and they have regular sales - I just stocked up when there was a sale on. You can also collect in store for free rather than paying a delivery charge, and if you want to return anything you just take it back to the store
H&m have a few bits but not much. asos can be quite good. Topshop have maternity wear online. It's difficult though, most of maternity wear is either frumpy and boring and horrible colours, or very expensive (Isabella Oliver).