If the company has a "young" feel to it, a black dress is rather funereal in summer. I think navy and white is a bit more forgiving and chic. Shops like Hobbs have some very good dresses available and if you go on a quieter day and explain your dilemma, an assistant will help you. They also do fluid trousers that hide larger thighs and matching jackets. You could then go a bit more interesting with a blouse and accessories. Jaeger sale is starting soon and they have really smart pieces for the office.
If you look at Hillary Clinton (yes, I know she is older) she does a really good job of masking a slightly larger backside. She is regularly in trouser suits with longer line jackets. You do not want loud colours, but you need something that says you are professional and smart so go for something that says this about you. Good quality shoes too!