My girlfriend lives in the Ukraine and she wants to come over to Scotland to stay during the Summer. We are under the impression that I need to send her some sort of 'Letter of Invitation' so she can get a visa. Any advice on what this should look like? Furthermore, if she was was going to stay in here with me after the Summer, how could she do this? Would we need to become engaged and apply for a settlement visa? Would we need to do this sooner rather than later?

Hi Bred,
I think it's unlikely that anyone here can answer your question as these forums are only concerned with family based immigration to the USA.
You might find what you need here Home Office
Good Luck

Ok, thanks anyway. Just to add though, she would like to do her Masters staying (and studying) in Scotland next year and just go back to Kyiv to take exams. Perhaps we will concentrate on getting a visa so she can stay for Summer first and when that is sorted reconsider our options for the future.