Thanks for the suggestions Whirlygig and Keef, but I haven't found a better way than the
VB script above. I can still enter times in hh:mm format and once setup, it's all painless from there on.
I'm surprised this question hasn't come up on this forum before - doesn't everyone use a spreadsheet for their logbook? But I guess it's only relevant to those recording times in hh:mm format.