You can get the fitting for your roof vent from a good plumbers or builders merchant. You will need to remove a slate or two to fit then replace the others over the flashing. Similarly you can get the cap for the top of the flashing vent from same source. Any good in line extraction fan should be able to run either on a horizontal or vertical situation. Get one that runs through a relay from your light switch with a timmer fitted so starts with you putting on your light and then stops after a predetermined time set by you to suit your bathroom after the light has been switched off. Preferably using a low voltage supply.
If fitting above a shower you can get some nice ones with a light mounted in the center of, Have a look at my favourite book, screwfix as you can see some of the stuff thats available and it's on line.
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