OH SNAP mate I am so sorry I didn't realise I have the LG 25UM65 not the LG 29UM65-P!! My Monitor has a displayport input at the back!!
HDMI did not get me to the correct resolution at all for Mac, no matter how much I tried.
I just looked for a HDMI (Male) to Displayport (Female) adapter but I dont think it exists...theres one on Alibaba (Ecommerce website) but not sure how reliable the description is because from what I've just read it needs some sort of converter.
I think your only option is SwitchResX (the program which you can download here http://www.madrau.com/srx_download/download.html) which I couldn't get working but I am not a hardware tech guy at all so apart from a few guides and instruction on this thread (first page) I couldn't get it to work properly.
Theres also an app called Display Menu (https://itunes.apple.com/en/app/display-menu/id549083868?mt=12) but I'm not sure if it allows you to set up custom resolutions.
Other than that I recommend returning the monitor and getting the LG 25UM65 and using the fix I used. But I might be wrong as I said I am not a tech guy so perhaps someone with better knowledge can give you better advice.
Really sorry mate I know how frustrating it is as I went through it myself! But worth it once you get it working!
APPLE NEED TO TAKE NOTE AND MAKE ULTRAWIDE RESOLUTIONS AVAILABLE AS PART OF THEIR MAIN SETTINGS!! IT WOULD SAVE LOTS OF PEOPLE LOTS OF HASSLE!!!!!