Answer (with thanks to Chris Bennett, county archivist for Hertfordshire): many Record Offices apply a blanket 100-year rule on their holdings, but in practice disclosure of information from FWW logbooks is a low risk. If worried about using such material, consult the RO; sensitive information can be anonymised. Before reproducing a complete transcription check with the current owner of the source. Always remember to include a REFERENCE to material, whether a direct quotation or not, so that others can follow in your footsteps.