You couldn't make it up!!

I think it may be a tad worse than that. Heathrow Express site shows
the last train at 23:48 from T5 as you said. But both TfL and Realtime
Trains showed a departure from T5 at 23:58 (to connect with the Heathrow
Connect at 00:07). Just a "shuttle" between terminals?

--
Robin
reply to address is (meant to be) valid