Hi Chris,
Thanks for all your help.
Deleting the maps did not work. However, after it froze I just left it for a while (probably about an hour) and it suddenly came right. I suspect something must have been taking some time to load. Keep in mind I'm not sure when during that hour I left it that it came right - possibly it was only a few minutes and I am just impatient .
I will let you know if next time it is any faster - perhaps it is a hardware related performance issue? I am using a laptop, but it is reasonable in terms of it's performance.
Specs for your reference: 5.9/7.9 windows experience index, 4gb Ram, Intel Core i7-3520 CPU, AMD Radeon HD 7570M Graphics. It plays most games okay (GTA IV, Civ 5, Skyrim, Anno 2070 etc.)
Hope this helps others.
Cheers,
Sean