MS has a history of misplacing things.
Hate to break it to ya, but Portland is actually a Heliport.
If you search wikipedia ( http://en.wikipedia.org/wiki/RNAS_Portland ), you will learn that:
RNAS Portland was an air station of the Royal Navy, built in 1917 as HMS Serepta, within the confines of Portland Harbour. Seaplanes were originally operated from the base's slipways. In 1919, 241 Squadron was disbanded and aviation operations ceased.
In 1946, Hoverfly R-4Bs moved in and the base's playing fields were taken over as a landing ground and became the site of the modern heliport. 815 Squadron flew its 12 Whirlwinds in from RNAS Eglinton on 14 Apr 1959 and the station was formally commissioned as HMS Osprey on 24 Apr 1959.
The base was gradually improved over the years, with the addition of a (short) main runway and landing spots. It has been the home of the small ships' flight Wasp, Wessex and Lynx fleet and at one point, the slipway was used by hovercraft.
As the base wound down, Flag Officer Sea Training (FOST) moved to Plymouth (HMS Drake). The Royal Navy closed HMS Osprey on 31 Mar 1999.