Originally Posted by Pargeo

Another thing - if you don't have any bulky luggage requiring lifts, then, when at Heathrow Terminal 5, take the LAST carriage and, when at Kings Cross St Pancras, alight through door # 2 - the exit towards the station will be in front of you. Just turn right, get the escalator up, pass the ticket barrier, then turn right again and then right again - this way is quicker than the advertised exit towards Kings Cross station - and this way you'll get to the station near its ticket barriers.