We have an overnight at Heathrow, arriving American Airlines late at night Terminal 3 (11 pm), staying at the new Sofitel attached to Terminal 5, as we depart the next morning on British Airways Terminal 5. I have made flight connections between Terminals 3 and 5 and vice versa but realized I didn't know the best way to leave Terminal 3 and get to the hotel in Terminal 5 as we are of course not on a flight connection and wouldn't think we can take those buses but will have to go through immigration and customs when we arrive and come back to the airport like a new passenger the next morning. Also, I don't see Sofitel on the Hoppa bus system. Would we maybe use the free part of the train system between terminals like you do if you are taking the tube from Heathrow into the city?

follow the signs for Heathrow Express and take that to T5 - it's free

It appears that the last Heathrow Express train of the day departs T1/2/3 station at 11:41 pm. It wouldn't take many misadventures of the incoming flight (late arrival, gate malfunction, a few passport control officers calling in sick, slow luggage delivery) to cause the OP to miss the last train. Is there a plan B for such an event?

My plan B if the flight is so late that we miss the last train -- we will have only carryon luggage -- then is to ask at immigration or baggage claim or phone the hotel. Thanks everyone.

Why don't you email Sofitel Heathrow Terminal 5 and ask them what is available when the rail service close? Both the Express and Connect services link the terminals.

I wouldn't bother asking at immigration or baggage reclaim - they won't be the slightest bit interested in helping you. At best they'll just refer you to Airport Information Desk (IF they're still open). If you can afford to stay at the Sofitel, you can easily afford to get a cab from outside the terminal over to the hotel if other forms of transport have packed up for the night.