Yesterday i found a place where i could get a short piece of chimney pipe just the length i needed.
Today i cut the hole in the wall and fitted the pipes through . When the outside pipework has been done a collar will go over the pipe and hide the gap round the pipe. There has to be a gap all round the pipe to stop heat causing damage to the plasterboard.
The plasterboard is not yet fixed as the wall behind has yet to be insulated and then the plasterboard can be finished.