I arrived back on Monday to find out that my sprue location was going to make it impossible to cut the mold. Katy suggested a single wire, of much thicker wire (3mm) placed on the back of the master pattern and bent at a right angle.
Unfortunately, soldering this new sprue in place, meant that my stone setting tabs fell off. However, I found that soldering a full loop in place, and then cutting off the excess was a much more reliable method of fixing the tabs on.
Once sprued up, I could start polishing. This little brush proved very useful for getting in the nooks & crannies.