Blocking up a chimney breast

[grazing cows]

Hi
We have a disconnected gas fire set into a chimney breast. We want to remove the fire which was disconnected at the wall by British Gas. We also want the the fireplace blocking up and the wall making good.
Are there major issues to be aware of when doing this?

 

[opps]

You may need to install an air vent in the opening to prevent damp and you may need to fit a cowl over the chimney pot to reduce the amount of rain water that can enter.