the only practical option is: Board the express coach at Stansted, get off at LHR. 5 hours is doable but cutting things awfully close IMO.

The OP is going from Heathrow TO Stansted. If you are arriving on an international flight and have to go through Immigration, plus collect luggage, I would seriously consider if you can make this ongoing flight or not.

Have you looked into the rules and regs of the ongoing flight. Some of the airlines are extremely strict about check-in times - and even if you are line (but not checked in) at the required time you are considered a no show.

Also, you don;t say what time of day or day of week you will be arriving - and this could make a significant difference in the amount of traffic.

I have never made this specific connection - but I would provide as much info as you can so those with more detailed knowledge can give you a realistic view of your chances of making the flight.

Actually, I just checked my times and I forgot I had to book a different flight, I actually have way more time. I get into Heathrow at 9:25 am on Monday, May 28th and then the from Stansted to Pisa at 6:45 pm.

Get a room in London near Victoria - it's easy to reach from Heathrow and close to coach services to Stansted. Then enjoy yourself up till around 3:00pm on the 28th.

I think no need for a room, same day flights, right?

With the times that you give, you'll have no problems with the transfer.

It seems there's a direct bus

http://www.londontoolkit.com/travel/heathrow_stansted_transfer.htm

Come to think of it, depending on your luggage, using the tube is not impossible and will be a bit cheaper, possibly a bit quicker: Piccadilly Line to Hammersmith, walk across the platform for the eastbound District Line, change at Westminster (down escalators) for the Jubilee Line to Stratford, then (escalator up, stairs down, I think) bus A9 to Stansted.

Nicole, weren't you going to fly to Athens on exactly the same date, and you also had already booked the flights, as per your other thread?
You got me a bit confused as to what your plans really are.

No need for a hotel as both flights are on the same day. I just have one bag, so the tube wouldn't be impossible, do I pay for the transfers? If it's about the same price, it would probably be easier to just take the bus.

Clauser, yes! I was flying to Athens, but switched my itinerary completely after getting a lot of helpful advice on the forum here. It seemed as thought 6 days in Greece really wasn't enough to see what I wanted to see, so I canceled my flight and booked a new trip to Tuscany instead. That's why I always buy refundable flights/trip insurance because you just never know! I now am flying from London to Pisa and spending time in Pisa, Florence, and Cinque Terre where I'll have time to go from place to place and see lots of things with little travel time.

oops--got the airports wrong way around. But the same coach goes the other direction too

You'll probably be at LHR at least a couple of hours, and you should be at Stansted before 5PM. The budget airlines are VERY strict about check in times, so I'd want to be there by 4PM myself.

So say you are through immigration before noon -- you are an hour+ from central London so 1:00-ish. You must be at Stansted in 3hours so very little time to see/do much.

If it was me, I'd just take the coach stansted and be done w/ it.

Yeah, I'm just going to go right there to the airport. Where do I get the coach at that goes to Stansted and how to I buy the ticket?