Hurrah! It works! Tatiana Yarmola, turns out the problem was that you can't link to anchors in a page whose title is a page id (bunch of numbers) rather than w proper title made or words. Aaaaand, lots of the GSIM pages had dodgy titles because they were direct imports from Word documents, which apparently contained some sort of hidden special characters that were not allowable, so the page titles were things like 776589356. Now I've removed all the special characters, so the page titles are proper, so the linking to anchors seems to work. Phew!