Who can replace rotten structural bay window timbers

We have just moved into a 1920s semi with mock tudor facade. It has a square bay with substantial wooden pillars at each corner upstairs and down. It originally had metal single glazed windows (crittall I believe). The timbers have rotted and require replacement, also there may be other problems due to the windows being replaced with UPVC double glazing, it looks like the lintels above the windows are sagging. This was all picked up by the surveyor before we moved in. What would be the best course of action to fix these problems? The surveyor suggested replacing the pillars with brickwork as a start, but I wonder if something more complex is needed. Photos at: postimg.org/gallery/1wob21w5o/d359b96a/