I'm not sure if it is illegal to leave you alone in your office, but it sounds unsafe. Check your contract of employment and see if the issue is covered, then have a word with your boss, and explain how vulnerable you feel under these conditions. If you get no joy, have a word with your local Citizens' Advice Bureau about your rights under current Health And Safetly laws. If all else fails, contact a solicitor - most will offer preliminary advice for a low, or even no fee. Are you a Trades Union member? If not, join the appropriate Union, and ask their advice - they can assist, and often have access to free legal counsel as well. Good luck - please let us know how you get on - it may assist other Users in a similar position.