to plug up a disused flue?

Unless you are prepared to climb on to the roof to put a cap on in place of the pot then its a profesionals job. Get Three estimates. And if in UK ( its a world forum) check with your local planning department if the flue needs to be lined as its only an electric fire.
Why? to stop you being conned by builder/roofer saying it needs lining. However if you think you might change later to a gas fire then it WILL require lining and the flue/pot capping replaced for a suitable chimney/liner type cowl.

bob
June 2012