And from the 'What Aerodrome' thread (p169) what it actually did look like:
Talking of the tunnels, as a schoolkid I often went spotting at Heathrow around 1960 and I can confirm what Overfly said that both of the two smaller tunnels did have raised walkways next to a railed off cycle track. I also recall that looking north from the Central area, just to the left of the main tunnels there was a tunnel stub as if boring had started but abandoned. Anyone know what the original plan was?