Just found a trick which lets you try Spotify Premium, which includes Spotify on your Mobile, for £4.99.
You only get 2 weeks, so it isn't really a saving as such, but this is a cheaper way to try out the service if you want to see what its like, since Spotify do not offer a two week subscription option.
First sign up for Spotify UNLIMITED, which costs £4.99 for a month, but does not include offline playlists or mobile usage.
After signing up, go to your subscriptions page and click upgrade to Spotify premium - this will upgrade your account to premium, but will half the length of the subscription.
After doing this you may wish to turn off auto-renewals to ensure you don't get charged another £9.99 in two weeks.