Secretaries of gas companies were frequently independant solicitors (and directors) and not always directly employed by the company. If you can find the solicitors he worked for, their successors may have information. The solicitors may have deposited material in the local county archives. Alternatively, as Salisbury was a large concern he may have been a member of the British Association of Gas Managers, later the Institution of Gas Engineers, (now with Managers added!). There may be a slight possibility they may be able to help, website -
http://www.igem.org.uk/company/contact.asp. There is also a link to the British Gas Museum on that site as an alternative. Finally try the National Gas Archives. Unfortunately, the archivist plus one other, are the only active employees. They depend on volunteers to a large extent, so expect some delay, unfortunate, but a result of the times.