Fireplace Coming Away From Wall in 1890 Terrace Property General problems, solutions, information and advice on fixing to walls of all kinds

[Scooby10]

I live in an end terrace (built in 1890) with two open fires. I have noticed an ever growing gap between the fireplaces and the wall they are fixed to. Is this a problem with the actual fireplaces themselves or something more worrying like movement in the gable end wall or even worse subsidence.