Hi
I had a very similar problem a few years back
took about six guys to look at it and they all gave different reasons for the damp walls.
chances are any rain that came down the chimney would not make the wall damp providing the chimney was vented.
my dampness turned out to be the lead work around the chimney which has split and caused damp in the wall. i have a 9'' solid wall and all the plaster inside and the render the house needed to be replaced.
I first thought it was the lead work but the guy who can first to look said i had soaker tiles and the what lead was there was fine!! although he did re-render the chimney as some render was loose!
problems continued 2 guy vented in the room, 3 guy vented in the loft
4th guy said it was condensation
5th guy said start from the top, replaced all lead work. walls started to dry out, but by this time plaster and render had blown.
6th rendered and re-plastered.
If you are hearing the rain, this could be in the from the gutter, I would go in the loft when it rains and check where the wall meets the chimney if you can see it, could be the sides of the chimney either lead gone or fillet (think thats what they call it)
There was something on TV a few months ago about a machine which can anaylise a scrapping of plaster and confirm if the mosture is rain water or condensation, in this program a guy called from the council with the machine, could be worth checking with the council.
my option get several guys around and get quotes, mine varied by considerable amounts and some quotes were in the £1000s!! and that was just the lead work
good luck