Is there time for a quick visit into London with a Heathrow layover

Yes! Take the Heathrow Express into town. Since you know your date and can estimate your time you can get an advance ticket for a great price. The last HE train back to Heathrow leaves from Paddington Station at 11:25 PM. If it takes you an hour / 90 mins. to get out of the airport (you'll need to go through customs; I hope you're bags are checked through to your next flight so you don't have to story them) you can be in town by 3:30 or 4. Gives you a few hours to walk around, see a few things and then get back to your hotel so that you can sleep for a few hours before having to go back through security very early in the morning. As far as safety, I would absolutely do it if it were me. Just exercise normal precautions as you would do in any big city and you should have no problem at all. I've taken the tube very late at night in London and had no issues at all. If you're uncomfortable with taking the tube back to Paddington from wherever you are, then just hail a cab to take you there.
To save time, get to know the tube before you arrive so you know exactly how to get where you want to go once you arrive at Paddington from Heathrow...otherwise you'll waste valuable minutes studying the map and trying to figure it out once you get there.

Thanks Anita. I just had someone else suggest staying in a hotel in Paddington rather than staying by Heathrow. The only disadvantage to that is that we'd be hauling our luggage on the train to Paddington. But then if we stay by Heathrow, we'd want to drop our luggage off at whatever hotel we were staying at.

I would give preference to a hotel very close to Paddington station. This would save wasting time with several journeys on a hotel shuttle bus to/from the terminal. You will be catching the Heathrow Express regardless, just take your luggage with you.

Here is what I suggest. In order to save time getting into London, take your luggage with you and check it in at the luggage storage at the front end at Paddington Station. This will save the hour or two you would need to get to your Heathrow hotel, stow your luggage, then return to the airport to catch the Express. Or, reserve a room in a hotel near Paddington, leave your luggage there, then hit the town. There are numerous small hotels on the square a block from the train station. There is an EasyHotel there, as well. It would be great if you could check your luggage thru to your destination so you wouldn't have to deal with it in London.

I would suggest spending the night in Windsor, which is a 15 minute taxi ride to Heathrow. You could take a late train from London Paddington Station (10-11pm). There is a hotel Ye Harte and Garter right next to the train station in Windsor.

If you are flying through terminal 5, I suggest staying at the Sofitel Heathrow as it is at this terminal, essentially, so it is a breeze to check in and then catch the Heathrow Express into London. It will just take a few minutes and then you can loll in bed in the morning as there will be no commute to the airport.

Since it sounds like you are at the end of your trip then try this...a little unconventional but I've done it in a pinch and it works...not ideal but it saves time of which you have VERY little!
Check your luggage through to your next flight but keep a bag with you that's big enough for what you would normally carry with you (wallet, phone, etc.), plus a change of undies, a hair brush, and a toothbrush. You can easily do this with a purse that's on the larger size. If you don't mind the slight "ick" factor of wearing the same clothes in the morning to get onto your flight then it will save the luggage hassle and give you more time in town. You can still take a shower, brush your teeth, etc., and you won't have the inconvenience of shuffling luggage either to storage or to your hotel. To maximize your ever so short time in London it might be worth it to try this??

I'm pushing the like button for Anita's suggestion!

I'd like Anita's suggestion too if I thought I could check bags in the afternoon for a flight departing the next morning. Never heard of an airline allowing that. Wray's Sofitel suggestion (or another hotel close to the airport with shuttle service) looks better to me.

If you can check or store the bags at Heathrow or a nearby hotel, then you have time to get into London that afternoon and evening, get back for a good night's sleep, and make your AM flight without problems. That's how I'd do it.

I doubt you will be able to send your luggage through, unless you are on the flight.

i haven't called British Air to ask them yet, but I'm thinking our luggage will get checked all of the way through so we won't have to worry with anything except what we want to carry on. We didn't book an open jaw trip requesting to stay the night in London. It's what was available when we booked (we used FF miles). So usually when you check in on the first leg of your trip and you're checking your luggage through to your destination, they're not looking to see how much time there is between your flights and saying you can't send it to your destination.