Yeah I've heard that too, but seems very far out! It would also need billions spent on infrastructure, whereas something along the M4 corridor would already have the rail links, or require only slight additions to either Crossrail and/or the West Coast mainline. LHR is on the right side of town, but should be outside the M25 rather than inside it! Removing LHR traffic from the M25 would solve most of LHR's access issues.