Best way of insulating old roof from inside

[Tinker Bell]

I've got an old cottage that was reroofed three years ago. I think that as the weather was showery when it was being stripped that the fibre glass insulation got wet/was not present. What is the best way of bringing it up to standard? The roof timbers are about 5"x3". Inside there are some fitted units. Regardless, I want to do it right as the upstairs is so cold. After the rads go off you can almost feel yourself bristling.