Never been there before, but we're coming back to the UK at Easter to visit family.
How long does it take to walk from arrivals T3 to the train station, and how long does it normally take to get through passport control/immigration? When we've come through the London airports it always takes forever. We won't have hold baggage, just cabin bags.
Passport control isn't usually too bad if you are a UK/EU passport holder in my experience. Never have to queue for more than a few mins. No idea about outside EU.
It is miles from T3 arrivals to the train station. I am a fast walker and often do T3 to train station alone with little luggage and it is probably 15/20 minutes, so at least an actual mile. If you have luggage, DCs or mobility problems, it could be half an hour.