I will be doing this transfer Sunday morning (JFK>LHR T3, LHR T5>PMO). We have 1:55 between flights. Supposedly 1:30 is enough time so I am hopeful. Found this video of the transfer in realtime, which eased my anxiety a bit.

One ticket or two? If on the same ticket and IF you arrive on time (important IF) you should be fine.

1 ticket, no checked bags.

If your incoming flight is not delayed you should be fine. Make absolutely certain ALL of your liquids are in a sandwich sized baggie. If anything is left out of the baggie or if you bring a bigger baggie you could be held up in security. I brought a quart sized baggie and they handed me a smaller one, and I had to re-pack it.

Update: Our flight arrived as scheduled. We did the transfer in 50 minutes. Luckily there were no crowds at the checkpoints or for shuttle buses. We each had our quart size bag of liquids, had no issues with security. We just followed the purple signs.