Cheers Guys
I just went with the large hammer option and after 1/2 an hour of
bashing it gave in.
So now I have a lovely flue hole (I'm 3 stories up)..
So I want to seal round my flue, I dont want to have to get scaffold
put up to let me cement round the hole.
I was just going to get a pipe and fire expanding foam stuff down the
outside of the flue till I see coming out the exterior.
Is this a goer ?
What other options are there, apart from do it properly ;-)

Cheers