SDK - li serve-sass broken with latest skin

I've noticed that with the latest updates to the Responsive skins that the SDK no longer works properly, we cannot get li serve-sass to function as it looks for a file that isn't downloaded by the plugin.

The result is that our workflow is now broken and we can't compile the skin locally, I've tried installing the SDK on a fresh machine with a new skin created in Studio and replicated the problem.

When we run li serve-sass for the first time we get the following error:

[08:42:33] Loaded gulp tasks in: 260 ms
[08:42:33] Using gulpfile ~/Git-Repositories/responsive/gulpfile.js
[08:42:33] Starting 'watch-res-sass'...
[08:42:33] GETing responsive options
[08:42:33] Finished 'watch-res-sass' after 260 ms
[08:42:33] saved configs/responsive.conf.json
[08:42:33] No directory found under coreplugins for res/feature/responsivepeak/. Returning default
[08:42:33] No directory found under coreplugins for res/feature/responsivepeak/. Returning default
[08:42:33] No directory found under coreplugins for res/feature/responsivepeak/. Returning default
[08:42:33] No directory found under coreplugins for res/feature/responsivepeak/. Returning default
[08:42:33] No directory found under coreplugins for res/feature/responsivepeak/. Returning default
[08:42:33] Server started http://localhost:9000
[08:43:04] core plugin downloaded successfully

however the service doesn't start and loading http://localhost:9000 in a browser window just gives a blank page with "cannot get /" instead of the usual structure to navigate to the CSS output file.

If I stop and restart the li serve-sass command the following error is returned:

[08:47:55] Loaded gulp tasks in: 259 ms
[08:47:55] Using gulpfile ~/Git-Repositories/responsive/gulpfile.js
[08:47:55] Starting 'watch-res-sass'...
[08:47:55] Finished 'watch-res-sass' after 455 ms
[08:47:55] Server started http://localhost:9000

events.js:141
throw er; // Unhandled 'error' event
^
Error: res/skins/responsive_ga/sass/skin.scss
Error: File to import not found or unreadable: responsive_peak/sass/vendor
Parent style sheet: stdin
on line 12 of stdin
>> @import 'responsive_peak/sass/vendor';
^

at options.error (/usr/local/lib/node_modules/lithium-sdk/node_modules/gulp-sass/node_modules/node-sass/lib/index.js:292:26)

it appears under the structure changes it cannot see one of the core vendor sass files and fails compilation on all machines we have using the SDK even with new skins created using the guide at https://community.lithium.com/t5/Responsive-for-Developers/Migrating-to-Lithium-Responsive-GA/ta-p/252459 

Has anyone else had this or a similar issue with li serve-sass not working? We really need this sorted as it's stopped our workflow entirely.