Thanks raymo.
Looks like it's a quick check with the multimeter and then a trip back to Jaycar, then.
Hopefully they have a replacement in stock that does put out a proper 12v.
UPDATE: Tested the unit with a multimeter and getting a solid 12.3v out all the 12v outlets. So it's the SkyWatcher power cable that's resulting in a ~1.8v drop. Next step is to replace it with something a bit beefier and see how that goes.