Hey MartyUK -
My apologizes for not getting back to you sooner. Let me run through a few different options to try to get you back up and running again. Try each of these separately and in order and please if you still have an issue or you find a particular step helped you fix it let me know -
1 - Go to your Swarm Cache Folder Location (there are possible two) and delete them:
Location One: C:\Users[UserName]\AppData\Local\UnrealEngine[VersionNumber]\Saved\Swarm\SwamCache
Location Two: [Unreal Install Directory]\[version number]\Engine\Binaries\DotNET\SwamCache
Your engine will rebuild these directories on the next launch and hopefully solve your problems.
2 - Temporarily Disable any Antivirus Software that may be running.
3 - Does your CPU have a Name which begins with a number, if so rename to have at least one letter at the beginning.
4 - Does your Windows Folder Paths (like those referenced above) contain any non-Latin Characters? If so change them to remove those characters.
5 - Increase the Virtual Memory allocated to your machine from Windows.
Thank You
Eric Ketchum