I arrive at London Stansted airport tomorrow
Could you tell me what is the cheapest method of transport is to go
to Central London

It depends whereabouts you want to go. The cheapest is bus/coach at about
£10 single, but takes about 90 minutes and goes to Victoria. The faster (and
safer) method is train, which takes about 45 minutes, costs £13 single and
goes to Liverpool Street. Bear in mind that both stations are extremely
congested (IIRC Victoria busiest Underground station, and Liverpool St.
busiest Mainline) during peak hours
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