Could any one tell me what this is for please

[Sutol]

Its in the inlet after the filter and is a butterfly controlled by a pull under the dash.

The only thing I can think if is that it is to shut of the engine by strangling the inlet.

Cant see a reason for that though as the engine can be shut off by pulling the hand throttle back a touch.

 

[mrmikey]

Yep, I wish mine had one, it shuts off the air.
I've never had a runaway yet and hope I never do, in your case you have the means to shut it down should one occur.
A runaway is when you have some other means of fuel getting to the engine other than the designed for one. Obviously if the fuel is not getting to the engine from the injector pump you can shut it off till the cows come home and it isn't going to do anything.
There's numerous means of getting some extra fuel supply to the engine. Going up a steep grade will possably put too much engine oil on top of the head and could get sucked down the valve guides. Bad seal on a turbo etc.
Heres a video of a runaway Runaway Diesel Defender 200 TDI - YouTube , here's an explanation Diesel engine runaway - Wikipedia, the free encyclopedia ....Mike