There used to be a pedestrian/cycle tunnel open adjacent to the main tunnel, but it's closed for works at the moment.

You mean the one-lane tunnel to the east of the main ones? You'd be mental to want to walk or cycle that. Many years ago I had a driver who had claustrophobia and refused to drive me any further than 50m in. Long story, but I had to get out and hail a black cab at the old Concorde roundabout (not an easy thing).

I've never dared cycle it (and I've cycled some pretty dodgy routes), although I believe the footpath was separated by a barrier.

It is, and it's raised significantly above the street. But the exhaust and dirt whipped up in there. No thank you.

Meanwhile, in the Netherlands:

https://c1.staticflickr.com/1/32/601...ad78a2_z_d.jpg

The road traffic goes on a lower level, on the other side of the windows (which also protect you from the fumes).

How'd the driver turn around in the tunnel? Or maybe I'm misunderstanding something?

Those northern Europeans - always so dam practical. This would never happen in England - not without a 36-year consultation, celebrity protest and relocation of a newt colony or two. And let's not even started on the colonies of bats. And not the kind with wings. :D

I want to say Impossible - nless we are talking many, many decades ago. (It's had gates and signs for as long as I can remember.)
Or unless there were some works going on and nobody stopped you?

I've cycled the small tunnels many times. You just need to make sure that you cycle in the middle of to ensure no cars overtake you.

I wouldn't cycle the larger two lane tunnels though and do believe cycles are not permitted then.

The question I want to ask is

Why would you want to?

I was flying to Toronto on a cycling trip. See my sig :)

When I was a younger I cycled there to watch the planes at the queens building. I lived a short distance away in Langley.

Apologies, it was actually Finland. The Toronto trip was from T5.

I quite like walking, and I quite like Heathrow. I also like approaching places in an unconventional manner. The idea of walking around Heathrow is nice.

I think the tunnel used to be open. It's been closed to pedestrians (very clear signage) for all of the time I've used LHR in tbe past 6 years.