I think Qantas747-438 need understand that's what things cost in London I'm affraid. Why didn't you just get the direct coach from LHR for £12?
STNEx takes 45 min to Liverpool St, via Tottenham Hale on some journies which has good tube connections. Liverpool St is a very nice station, very good for the city, but miles from the West End where all the hotels are.... It runs every 15-30 minutes depending on the time of day, it runs on commuter lines which limits the trains WAGN can run.
There are various airlines that offer heavy discounts on the train fare, or agents at destination airports (the local coach firm in Stockholm sells STNEx tickets cheap for example).
Common STNEx mistakes to amuse the natives are catching the train to "Liverpool Lime St", several hundred miles North West of London and just sitting there for five hours thinking 'gees this is taking ages', or catching the slow local train and demanding your mponey back because it took so long and you missed your flight.
To be honest the land links at STN leave a bit to be desired, the airports growing at 25% a year or more and they're not keeping pace.