Heathrow Express, typically one of the most expensive railways in the world, is doing a special summer off-peak for £15 one-way or £25 round-trip.
You can book on the website, and use trains except those on Monday to Friday, 7am-10am and 4pm-7pm. All trains at weekends are valid. Book at the Heathrow Express site.
Still more expensive than the Heathrow Connect https://bugadvisor.com/2017/05/21/heathrow-connect-avoid-the-heathrow-express-rip-off/
Thanks for sharing this tip and to Paul for highlighting alternatives.