I'd say try New Look. I know its probably considered a teenage shop but they do have some classic tops and trousers in there at really reasonable prices.

Also if you have a Sainsburys, they also have nice tops and trousers/skirts all at about Ł15 to Ł30 an item.

M&S have some Ł15 linen trousers at the moment which are cool for summer but do crease, but then, that doesn't seem to matter too much in the office nowadays!