I don't think that dress looks frumpy at all. Go for it. I would love to wear a dress like that, but I am too pear shaped 
"I find Boden dresses have too high a neckline for me (Size 12, big boobs) so look I matronly. They're too long (I'm 5'4") and finish unflatteringly just under the knee"
Being 5'7" with small boobs it sounds like Boden were made for me. Sadly most of their clothes are too £££ for my pocket.