Shower room Fan Ducting - Change from Soffit to roof tile exhaust?

Hi.

Finally getting around to looking at upgrading the fan over the shower from a basic (25l/s)( in line 4inch duct extractor mounted in the loft space, venting 3.5 meters away venting in the soffit, via flexipipe (uninsulated) with a 225L unit with timed run on & humidstat monitor and & insulated ducting.

To make matters worse, the flexitubing is "squeezed" to fit between the roof line and the outside wall to fit, so it's limited there as well.

However, I've had to install a roof tile vent for the kitchen extractor, and wondering wether I should go to the effort of getting someone back in to install another roof tile vent at £75 but allows me to go "straight" up approx. 1.3 meters.

If I go "straight" up, how do stop the natural chimney effect when the fan isn't on?

Thanks in advance.

You could use something like this:

http://www.vent-axia.com/range/back-draught-shutters.html

I did something similar at my parents' house a few years back. The instructions for the fan I used (Vent Axia ACM) stated that the fan should be at the highest point in the system to avoid any chance of condensation getting in to it. I managed to achieve this but still used insulated ducting.