I would like to know this too! Next used to be great for pear-shaped trews but not any more.
I've ended up with quite a few pairs of M&S cheapo straight-leg or bootcut jeans and cords recently, they are okay but not wonderful. The high-waisted ones actually fit not too badly on the waist, but the lower cut ones still gape at the back and require a belt to stay up.
I would try Gap Curvy but their website has hardly any sizes in stock so that's that one scuppered.
I've seen French Connection recommended before but they're a bit pricey for me and there isn't one nearby to try.
I had a style consultation the other day and she recommended buying expensive trousers in the sale and also getting trousers altered but to be honest I'd be reluctant to buy trousers and get them altered incase I still didn't like them afterwards.
Apparetly places like coast, phase 8 etc are worth checking out but I ordered a load of phase 8 trousers in the debenhams sale and they were all horrible
I have several pairs of the Gap long and lean jeans which I liked but when I went to get another pair recently they seemed to have changed the cut. I brought a paid of the curve jeans instead and have regreated it ever since - they are too curvey and seem to accentuate my pearness rahter than conceal it!