Another suggestion from Ross Moore works brilliantly. Instead of a protected blank in the 1st line of each minipage, instead put a TeX \null.
\null is a macro for \hbox{} which creates an empty box of zero
dimensions. This effectively forces the minipages to top align with the
very top of the minipage (rather than the baseline of a space with a
physical size.
I had never heard of this, but found it in Knuth's TeXbook (Appendix A,
12.6).
regards
John O'Gorman
