How do I stop condensation forming on the front of the glass?

If you move the under tank, heating mat/cable to the front of the tank, leaving the back unheated, this will solve the problem.  In hard water areas lime scale can be deposited on the glass.  If you only use cooled, boiled water this will not be an issue.

 My tank smells of rotting meat.

Make sure all food remains are removed from the tank.  If you haven’t seen your spider for a while it could be dead.