Heathrow Express Discount code for 50% off ticket is WP3828.

Gatwick Express code is WP1836.

The codes apply to single tickets only

BUT as others have pointed out, getting the HEX into town may not save you any travelling time as it's certain you'll have to get a bus / tube / taxi to anywhere interesting. In some cases you end up paying more money to take longer to get anywhere.

Allow at least 2 hours to get back to Heathrow - 3 if you need to recheck / get a boarding card - and allow a bit more time if, as seems likely, your return trip is anytime between 4:00pm and 8:00pm