It takes from 1hr 15 mins to 1 hr 30 mins from London Euston and if you book in advance you will get it cheaper. An open return is approx £30 currently from 01/08 onwards depending on what time of the day you go.
The best way to go though is from London Bridge. It takes approx 1 hour and you can get an open return for under £20 if you book in advance. That's what I normally do.
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