Where did you buy your ticket? Its not unusual for 2 people to pay different prices on airlines - while i don't know silverjet try ryanair , easyjet or even British Airways - buy your ticket one day its £75 the next £200 and then for no reason on a third day back to £100. All about yield management apparently.
More importantly, how was the flight? You were obviously happy to part with the £1200 before you found you could get it cheaper - so was it worth £1200?