Customs is where the big hold-up will be. I think it took me about 15 minutes just to get to customs, then another 10-15 standing in line. It'll probably take about another 10 to get through. Make sure you have your passport and other documents (flight info, etc) ready when you get to the counter. There's nearly 45 minutes just for customs, though it might not take that long.

They were great with my baggage, and I didn't have any problems. You'll probably have to receive your luggage, then check it again for the adjoining flight though, due to customs.

The tram between terminals only takes a few minutes. I'm wondering if they're estimating 70 to include the time in Customs. Who knows.

Honestly, I'd suggest looking for a flight with some more time between when you arrive and leave. Heathrow's a really nice airport, but it does take some time to get around.

Oh, another thing, make sure you read up on what you can take onboard the flight with you. It seems to change all the time, but was restricted to just one small laptop bag about a month ago.