A vague memory tells me that the cargo tunnel, which I think exited or maybe still exits twixt the corner of T2 and the T2 multi story car park, was open to the public for a few months while some work was being carried out on the twin tunnels sometime around 1970 ish. It was great for those of us who lived south of LHR since we could come into the central area from that side and save a lot of time.