St. Margaret's Bay is about 5 miles east of Dover. It's machine gun post seems to have been adapted during the war from existing tunnels. A visit to the nearby Coastguard pub, Strickly for research purposes you understand, proves that this tunnel shows up on many old maps and photos from the mid 1800s.
There was a small community living at St. Margaret's bay until it was taken over during the war by Royal Marine Commandos, who used the houses for urban warfare training, the buildings were eventually turned to ruins, never to be replaced.
The local pub was accidently blown-up by Canadian soldiers!

The embrasure is in a concrete structure jutting out from the cliff face, and the entrance which is on the esplanade has been bricked-up for years.

Here's my photos:
1. Looking out:

2. Looking in:

3. Turn right at the end:

4. And right again to see the bricked-up esplanade entrance:

5. Un lined parallel section

6. Here's the south end of the tunnel. Stairs leading to another opening 40ft above the beach. There was once a small room up here but it's now lost to the sea:

7. Back to the embrasure:

8. And back on the beach:

9. The bricked-up entrance on the Esplanade
With the machine gun post in the distance:

10. The Machine gun post.

11. A Pillbox next to the Entrance.

The machine gun post was manned by members of the 70th Battalion The Buffs until November 1940 when it was taken over by the 5th Battalion Royal Marines. The Buffs were then move to guarding the sites of the cross-channel guns, Winnie and Pooh and the radar station at South Foreland battery.