Friends, thankyou for the suggestions.

A slurry of sopmething or other to fill the voids sounds as if it is
the bnest option.

I was interested in the thought about clearances, the stove was fitted
by a qualified installer who also lined the chimney as approved by the
Planning Office. No comment was made as to the stove being too close
to the back wall but it is a possibility!

Thanks again, Peter.