Manrose CF100T  4 inch 100mm wall or ceiling centrifugal fan. Online special at just £22.25 ex-VAT.    

The Manrose CF100T is the centrifugal timer model incorporating an integral adjustable electronic timer which has a range of 1-20 mins. It is designed to be mounted on walls or ceilings and vented through a 100mm (4") diameter flexible or rigid duct.
Fan speed: 1600 r.p.m.

Extract rate 62m³hr, 17 litres per second - Fitted with a built-in  backdraught shutter.

Superior performance, British made and a 2-year guarantee.