chimney

hi, im trying to get my open fire up and running but am having problems with smoke coming back out into room. ive had a sweep round and he said there a blockage about half way up so ive made a small opening and found hollow tube in there about 8 '' diameter & about 12" long its is loose i can move it round should this be there?? also where the fire place was shaped inside thats been broken away (to fit gas fire). do i need to reshape this?

I'm trying to visualize what your describing would it be possible to post pictures ?. Is this pipe going through the wall horizontal or in the chimney vertical. If vertical it may possibly be part of an old liner or cowl thats fallen down.
The section where the fireplace has been broken away should be rebuilt to original dimensions as it is important to have a properly formed fire back and throat ( the area just above the fire and at the bottom of the chimney) as this will assist the smoke and gases to flow up the chimney smoothly and safely.
Im surprised the chimney sweep didn't investigate more thoroughly.

thanks sootylum, that pipe was running vert but have got most of that out now and seems to be much better. ill try and reshape chimney as that does make sense as the smoke seems to catch there and come back down thanks agian