Bodged conservatory leaking.Cavity trays needed

[benyule]

The previous owner installed a conservatory on the back of our house which leaks.
The conservatory doesnt have cavity trays in place and didnt even have any flashing when I bought the house.
I put up a flashband as a tempory fix which has improved somewhat.
The water is coming down through the bricks though.
How much of a job is it to put the cavity trays in?
How much should I be looking at if I was to get a local builder in to fix?
Many thanks

 

[foxhole]

Just need the wall above roof chasing to accept lead flashing , damp walls inside will take months to dry out.