If value is important, you can go via Tube/Train for £11.70 per adult (providing you pay by contactless card or phone, and do not use any train which has "Express" painted on ths side). Assuming you have collected your luggage and are leaving the terminal at 2230, you'll easily get to Blackfriars on the underground (one change at Hammersmith, cross platform, step free) before the tube shuts for the night. From Blackfriars the Thameslink trains run all night. Frequently until midnight. Every half hour from midnight to half one, then hourly throught the small hours.
Obviously nowhere near as convenient as a transfer, but a lot cheaper.