Agreed that this is the best option to get from LHR to the Andaz via public transport. However, HEX and the "Circle or Hammersmith & City" line do not share the same platform, far from it I'm afraid to say. In fact, they are linked via a covered walkway: when stepping off the HEX, walk back and up the stairs, then straight ahead via covered walkway to the Circle line station and then down to the tube tracks. Everything is signposted but as mentioned before, it's not just crossing the platform.