As others have said the pricing systems for airlines allow different prices at different times depending on loads etc, I have talked to passengers, not with Silverjet, sitting next to each other who have booked roughly the same time and have paid £350 and £750, just by booking on different web sites. I don't think there is anything illegal with this as it is the service you are purchasing and you have a choice!
Think of it being the other way round! You had paid £900 and found out that the person next to you had paid £1200, would you have thought that illegal! Would you have demanded that you both pay the same, even if it meant increasing your fare to £1200, or would have just thought "Hey I got a good deal"?