And good old ASOS is a one stop shop for blazers (and everything else), my fav is this lightweight grey waterfall blazer (£55), it's great for the office but would look equally good worn with jeans for a smart/casual look.
Or how about this season's boyfriend blazer (£60) in the go to shade of the season - Beige. great for more casual looks but will also work with your work wear, And as it's made from linen, so it'll keep you cool when the weather does finally hot up, making it a longer lasting wardrobe piece for the summer, so a great investment.
Or how about a classic white tailored blazer by Mango at £54, this is a definite summer wardrobe must have, as you can mix it with absolutely anything!
All from http://www.asos.com/