I was having some trouble with the MySQL setup and my host suggested that I reinstall WordPress from scratch.
I have three blogs in a network: electro-web.co.uk, photostream.electro-web.co.uk and miniblog.electro-web.co.uk
I reinstalled and retrieved the backups perfectly well and the main blog works perfectly. As do the home pages of miniblog and photostream. However, clicking links to individual posts on those sites just redirects to the homepage of the main blog. The addresses are right, it's just that something makes them redirect.