We were only in Stornoway for one night, we stayed here:
http://www.laxdaleholidaypark.com/accom/bunk/
Communal kitchen/lounge/bathrooms, very small rooms each with 4 bunks and a heater, but extra cheap.
There's also a circle about 1/2 a km away, and a shop/garage the same distance in the opposite direction.
It was nice enough, but would probably be a bit wearing if you were there for more than a couple of nights.