preparation for fitting of inset gas fire...

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we have recently had our gas fire removed to find the complete back boiler system still in place,after various quotes(some being extortionate)i have decided to try and understand what i am being quoted for,so here goes........the back boiler has now been removed and chimney swept,can n inset gas fire be directly fitted into the opening or do i have to have some sort of fire liner inserted or throat or backplate,the chimney itself is in good shape(as stated by chimney sweep bloke),so i guess i dont need one of those flue tube things,the opening itself is back to the basic fire pit as ive been told they are called,the internal hearth is still in place and in good condition,can anybody describe what work is required to finish job and rough calculation of cost to completion as i dont fancy being ripped off...cheers steve