Sorry for the late reply. I'm in Melbourne and posted quite late at night.
I understand where the advice was coming from now and can appreciate you were discrediting the test for my benefit, not to dismiss me out of hand. Andrew is right, the internet makes it easy to overlook things and I was mistaken when I perceived his response as criticism.
I appreciate everyone's advice. Thanks for the follow-up Andrew, I could easily have made the mistake of stopping down after setting focus and my new humble tests confirm something was likely wrong with my method as the Sigma performed a lot better. I have posted links to them below. The high resolution files can be downloaded from a link on the side. Unfortunately it's way too hot and sunny here at the moment to shoot a brick wall (30-43 degrees celsius the last few weeks), so I grabbed a random newspaper instead.
Please note these tests were with a standard Metabones Nikon to M43 adapter, not a speed booster, as I wanted to test the lens without additional glass. The adapter seems to cut a little light so wide open at F1.8 it is about as bright as the Voigtlander at F2 which is why the images are at slightly different apertures. Disclaimer: these tests are designed to help me make decisions about my kit because I have seen no comparisons with the Voigtlander 17.5mm. I hope they might be of use to others but they are not professional tests!
Test 1 focus was set to the smaller black and white group photo
http://i32.photobucket.com/albums/d7/lucki231/Test1Sigmaat18mmF18ISO400_zps9333702a.jpg~original
http://i32.photobucket.com/albums/d7/lucki231/Test1Voigtlander175mmF2ISO400_zpsa0b08ecd.jpg~original
Test 2 focus was set on the buckle of the bag
http://s32.photobucket.com/user/lucki231/media/Test2Sigmaat18mmF18ISO800_zpsaea451e0.jpg.html
http://s32.photobucket.com/user/lucki231/media/Test2Voigtlander175mmF2ISO800_zps1706c1d7.jpg.html
Test 3 focus was set on the middle of the green and white cushion. An attempt to replicate my earlier test. It appears there was something off with the earlier one as the Sigma is a lot better here.
http://i32.photobucket.com/albums/d7/lucki231/Test3Sigmaat18mmF18ISO400_zps99d8a5b5.jpg~original
http://i32.photobucket.com/albums/d7/lucki231/Test3Voigtlander175mmF2ISO400_zps779a4cf9.jpg~original
Hopefully these comparisons are of some benefit. Thanks for all the input.