I'm trying to record using an audio cable attached from my MIDI keyboard to my Scarlett 2i4 interface. The audio cable is fitted with two 1/4 inch adapters either side so it fits into the "line in" socket of my interface and the "phone" output of my keyboard.
 
When I try and record what I'm playing onto Sonar, basically, whilst there's a signal, there's a lot of interference (it's reduced a little by pressing "PAD" on the interface, but not completely).
 
Note that the audio cable's attached to the phone output of my keyboard. There are other options, like "LINE OUTPUTS" - R or L/MONO, and when I attach the other end of the cable to L/MONO, no keyboard sounds are produced at all and instead the recorded audio consists entirely of interference.
 
I really don't know what I'm doing or if there's an obvious answer to this so any help would be appreciated. :)

Use the line outputs from the keyboard.  To the line inputs of the scarlett, not the mic inputs.  If the scarlett has "combo" inputs for both mic and line, but sure to use 1/4 inch plugs coming from the keyboard into the scarlett.  This sounds like what is happening w/ your talk of interference.  Not sure what you mean, but plugging a line level output into a more sensitive mic input will overload the input.
 
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The audio cable is fitted with two 1/4 inch adapters either side so it fits into the "line in" socket of my interface and the "phone" output of my keyboard.
 
Why does your cable need adapters? You should be using one or two standard 1/4" TS unbalanced  ( Guitar) cables, Sounds like your using something funky and probably not shielded even. 
Use the line outputs of the keyboard and set the scarlett to line input. 
 
Look on the front-- see---      Line/Inst and a PAd jf the signal is still too hot.  
 
 read the manual that came with the interface, it's very well documented the correct connections for a keyboard, they even have a fairly large diagram!  
 
 

So it's the cable I'm using that's causing the problem? That I need a 1/4" and not just a 1/8" fitted with adapters to work like a 1/4"?
 
To clarify, the two line out ports I have on my keyboard are "R" and "L/MONO". I don't get anything but interference from L/MONO, I only get sound from the headphone jack. So, again, that's a problem with the cable being used?

Without looking at the outputs, inputs and cables, it sounds like you are using a headphone output from your keyboard.  Does the keyboard not have proper 1/4 inch outputs?  Give us a detailed description and ID your keyboard so we can look it up to help.  Traditionally, the L/mono output contains a mono version of the stereo output, although that is more common for inputs.
 
It sounds like (again) you are using a headphone output which is a powered connection, which is overdriving the scarlet input.  But it is hard to say from your description of what you are hooking up how.  As cactus says, there are different connections and different output levels.  It takes a while to suss them out. Also, your local music store will put a premium on cables - a lot of markup.  Once you figure out which cables you need, you can save a lot of money by ordering over the internet rather than GC etc.
 
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Boy I guess your not in a big hurry about this. 
 
I have patched lots of Keyboards. If you read your manuals it is very simple and well explained. I hate to keep saying this but the information you need is quickly accessible and very clear. 
 
Unless the keyboard is broken the audio from the L/R jacks will be the stereo sound of the keyboard, using the L/mono is the mono sound of the keyboard. 
When you plug something into the headphone jack it will kill the sound from the speakers ( in the keyboard) and possibly the output jack too. The output of the headphone jack will work but is not a good impedance match for a line input. 
 
The correct cables needed are 2 standard guitar( instrument) 1/4" TS shielded cables. $10 each at any music store.
My guess is that your cheapo headphone cable with adapters is creating the background noise as it has pour shielding. 
Set to switch to Line, Engage the Pad   turn the level all the way down, play the keyboard as you slowly turn the level up until the green ring turns to orange and back off a smidgen. If you find yourself having to turn the level past 2 0 clock then turn it back down, Un engage the pad, and start over. The goal is to have the green ring steady and not see the orange or red. 
 
AT the scarlett has line/instrument switches so it's easy to connect any device 
 

Also, you can youtube this for added visuals to see a step-by-step help guide. :))

Thank you for your help!