How to transfer from Terminal 3 to Terminal 5 at LHR

Even if you are changing airlines, is this on one ticket? If it is, your bag should automatically be transferred to your next flight. Then stay airside and you will be transfered to your next terminal. Follow the "Flight Connections" signs.

If this is two separate tickets, you will go through passport control, collect your luggage, walk through customs, take the Heathrow Express train (follow the signs) to Terminal 5, check in, go through security and find your gate.

This website should help:

https://www.heathrow.com/connecting-flights

We have even taken a taxi from gate to gate, a long distance at times.

We were expecting to have to do this in reverse last week (T5 to T3) but the day before our incoming flight changed from landing at Terminal 5 to Terminal 3.

We were going to catch the underground between terminals using our credit card to tap on and off and expected this trip to be free. We decided on underground rather than Heathrow Express as the trains are more frequent.

We landed mid afternoon at T3 and had cleared immigration and had collected our checked luggage within 45 minutes. We then had to walk out of the arrivals area and in the door of departures. So arrival to being checked in for our next flight was under 60 minutes.

We were arriving from Madrid on Iberia with our next flight to Singapore on QANTAS.

By the words "new flight" I am guessing you mean two separate tickets not bought together? Three and a half hours is not enough time to do what you would need to do. I doubt 6 hours would be enough. If your flights were bought as "one journey" and you don't have enough time BA will move your flight to a later time. If you bought two tickets that will not happen. I suggest calling the airline and seeing what you can do now to increase the layover time. LHR is huge with a massive amount of people moving thru it and way more security lines than any other place I have traveled to, which adds to the amount of time needed to get around. J

The landside connection route is either via the Underground or the Heathrow Express. The former will need use of contactless payment although you won't actually be charged. You can get a free pass for the latter. Underground is probably better as it isn't so deep and runs more frequently if it has a good service when you want to use it. I think it is doable but not of course without risk. Try to do online checkin and bag drop.

This is a very easy change to make and you have plenty of time if your flight arrives as scheduled.

Using the Underground is free, you need a free ticket from the machine for the other train options (Elizabeth line and Express). It takes 20 minutes

The person who suggests this takes six hours has clearly never done it. You need an hour maximum to get from plane to train. Sounds like you will have a short or medium haul second flight so only need to be checking in two hours before to meet airline guidance.

Thanks everyone for your help! I think I'll use the train between terminals as I'm more comfortable with airport trains & hope I can make the scheduled flight. If not, there's no penalty to move to a later one.