Need help to identify early 20th c Sub

Hi, I was wondering if some members more knowledgeable than myself could help me out with identifying a British sub that I saw on a program a while back that was sunk on exercise in the interwar years. A few years back I saw a program where some chaps were diving on a sub sunk off the British coast they even gave a brief history of it, but my memory fails me as to its name and class. I've spent some spare time searching the web with the details I can remember but I always draw a blank. the details I remember are thus........

The idea behind it was conceived in WW1 but i think it arrived too late for service.
It had a monster artillery gun mounted on it with the view to shelling lad from out at see and then making a getaway.
it was lost or so the program claimed whilst diving a ship collided it with it, the known facts being that it broadcasted its intended dive time and then was never heard from again.
on the program I saw the divers visited the wreck and the huge gun was knocked off its mount possibly from the bow of the ship that hit it dragging it to the ocean floor.

Hopefully from the above details someone can provide an identification and possibly some more history or at least point me in the right direction.
I'm pretty certain the facts I've given are correct or at least as correct as the program stated them but I can't for the life of me remember the sub. Like I said I've looked on the web and I don't think it was a M class I'm more inclined to think it was a 1 off for some reason though like I say my memory does fail me

Thanks in advance.

 

Maybe it was this one?

HMS M.1

 

that's the bugger, thank you very much.
HMS M1 - Wikipedia, the free encyclopedia
Now I really don't understand why despite never looking at M class subs I'd never picked up on it actually being one of them. For some reason all the pictures I found on the net didn't correspond with the picture of it I had in my mind,. However the above description fits perfectly again thank you, now off to bore my daughter with another military fact. I'm on a mission if I can get at least 5% of my interests into her memory before I forget it all I feel I'll of achieved something.

 

You´re welcome! And good luck for your mission!