Advice for dealing with possible back boiler in the fireplace

[northc1]

We suspect there could be a backboiler device fitted in our fireplace. Following heavy rainfall at the end of last yr it now gurgles very loudly when we light a fire and part of the chimney breast become too hot to touch (seems to be plasterboard rather than brick in one section). Could do with some help to remove or disable it to ensure the fire is safe to use - are there any local plumbers of builders who have experience with this kind of thing??

Thanks!