724 to Heathrow is now £12 single, which i think is a bit of a rip off, the Arriva Herts & Essex, All Zone ticket, isn't valid to Heathrow, but the All Zone looks like it's valid to Uxbridge (i thought it was Maple Cross), so change at Uxbridge for the TfL A10, Arriva All Zone day ticket £7.90, the TfL A10, £1.65, even returning on the A10 then 724 you're still saving money
£12 single from where?
If it's Watford or Rickmansworth then it's probably cheaper than the train.