Hey MartyUK -
A couple of other thigns to try out:
1 - Increase your virtual memory allocated to windows. (Tips to improve PC performance in Windows)
2 - If that doesn’t work you may also want to try deleting (or making - both have worked for different users) teh Swarm Cache folder which is in the \Unreal Engine\4.5\Engine\Binaries\DotNET\Swarmcache
I am talking with some of our engineers now to try to debug this issue.
Thank You
Eric Ketchum