Just so you're aware, Transport for London has effectively gone down the route of other cities by creating a surcharge on airport travel, in that Heathrow is charged at peak rate whatever the time of travel. It's a pretty significant uplift.

There is a way around this, because at the same time they put Hatton Cross inside the free Heathrow travel zone, which previously was solely the terminals themselves.

If you exit there and immediately turn round and re-enter, catching the very next train (with light luggage, I can do this turnaround in just over a minute) then you'll only be charged at off-peak rate from Hatton Cross. If you're visiting for the first time, with children and significant luggage, this probably isn't something I'd recommend though!

The increased cost will also count towards your daily/weekly price cap, so it might also make no difference to you if you're going to make multiple journeys.