Originally Posted by
Oxon Flyer
ICheapest is [snip] £5.50 single (contactless card) to Heathrow.
Or, if you really want to squeeze the price, hop off at Hatton Cross, go through the barrier, and straight back again to avoid the TfL Heathrow permanent peak surcharge. £3.40 off-peak, £5 peak, then the Hatton Cross to Heathrow bit is free on contactless.
It's the Heathrow version of the East Croydon trick going to Gatwick.