I want to open a specific file - if I press F5 or another key - its a file I use all the time, so want it to be open at a touch of a key
Create a service like this and give it a shortcut in System Preferences > Keyboard > Keyboard Shortcuts > Services:
There's a bug in 10.7 and 10.8 where the shortcuts for services don't always work until you hover over the services menu from the menu bar. There is also a short delay before services are run, and the shortcuts don't work when the frontmost application doesn't have a services menu.
I use Alfred:
In Alfred 2 hotkeys have a short delay unless you change the trigger behavior: