damp and structural worries where modern casement window fitted in outer leaf of cavity wall - ?

This was done (not very carefully) some years ago. The window is 6' wide - almost the entire width of the wall, with about 12" left (internally) on one side and only about 5" on the other. Support is a concrete lintel on the outer wall and 4"x4" timber supporting inner skin. Damp is one thing - there is no cavity tray and I am not sure if damp is coming via the cavity or being driven through wall around window frame. Also I am concerned that the window is so wide and wonder if I could tie in bricks on the side which goes closest to the return wall and narrow the window frame at the same time. Any ideas where I should look for answers to retro-fitting cavity tray and tying what would be just one brick pillar to the wall? The whole thing is a bit of a mess and I want to get it done in one go if possible. On the plus side no sign of movement in walls to side or above. Thanks for advice! Peter