No, LNG is Liquified Natural Gas. The detector will almost certainly work on butane but it may not alarm at quite the 'right' concentration. Since that is well below the lower explosive limit (LEL) it probably won't matter to much.
I suppose I had better have a look at it!
I see it also works for propane. I wonder why they don't say anything about butane.
Read the instructions carefully before testing sensors like this: some can be "poisoned" by the gas used to test and then fail to work when required to do so.
Just a quickie .. butane, propane and LNG are heavier than air, whilst natural gas as supplied to house is lighter than air ..
as it is mainly methane ...so when testing the maplin detector, you really need to test it with the correct gas