One can purchase West End theatre tickets on websites such as
Box Office, UK theatre tickets or Theatre Tickets Direct. One can
also buy tickets on Last Minute and London Theatre.
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Cheap theatre tickets in London can be purchased through a
variety of websites. These websites include Viator, the Official
London Theatre website, National Theatre Travelex Tickets, and Get
Into London Theatre.
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it depends. what theatre and what play?
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In the this sentence, You called the theatre about tickets but they didn't answer, has an incorrect reference. "They" currently refers back to tickets.
To avoid repeating "theatre" twice, rewrite the whole sentence.
You called about tickets, but the theatre didn't answer.