That has seemed to do the trick, started a new game with nightly 2015-07-27-1005, no Mechtoids, though I have to use the nightly exe rather than extended v2.4 (was looking forward to 4 weapons on a craft and the training room facility) but I'll maybe post a message on that thread as it's a bit weird but I'll not complain too much. It works and I can assault the base, probably get creamed but that's half the fun. The terror mission I'm doing at the minute is "alens attack infrastructure". Should be a hoot as its sectoid, floaters and both their terror units.