I'm closing off my chimney, do I need to have it swept first?

Yes! If a chimney is closed off then it can get damp. If there is any soot inside the chimney then this could turn into a black, acidic slurry which could penetrate into the internal walls causing severe problems. If bad enough, this damp can penetrate through to the adjacent room and cause unpleasant stains on the wall.

Chimney cap Suffolk Essex