Thank you for your question.
Unfortunately you wouldn't be able to fit this stove with a back boiler in the fireplace. The back boiler will already have a flue system inside the chimney which would need to be replaced to fit the stove.
We would recommend calling our stove experts on 0845 450 8351 for fitting information or getting some advice from a Hetas registered stove installer (Find an installer here http://www.hetas.co.uk/find-installer/) before purchasing your stove.
Hope this helps?