You tell me that the chimney is sealed up but not how it has been done. If it is just covered up with a piece of hardboard it would be an easy matter to rip it off with a claw hammer or something similar. If the chimney is bricked up, though, you will need to use the services of a handyman or bricklayer. It is a simple job to form the opening for the flue and you should expect to pay between £80 to £100 for the work.