In the end I went to this page and downloaded the super patch - I had tried to download a superpatch from here but I just kept getting a 403 error message.
I'm not sure if it was just the super patch that resolved the issue or a combination of things so I'll tell you everything I ended up doing:
1. I turned off my anti-virus (was still unable to install 1.67)
2. I had found a patch version 1.50 and I was able to install this one (my version was 1.42) so this made me think perhaps I needed to upate incrementally manually
3. I installed patch version 1.55 but still unable to install 1.67 so I went hunting for some more patches in between 1.55 and 1.67 that's when I found the page with the super patch
4. I installed the super patch as advised on that page but then was unable to load the launcher
5. Rebooted my PC and hey presto, although now when I click the Sims3 icon it loads Origin and from there I go to my Games and click Play on Sims3 🙂
