Prefabricated flue query

[staffordian]

Our home is a 1973 built bungalow. The lounge has what I believe is a prefabricated flue which currently houses a shallow inset living flame gas fire, installed before we bought the property.

What leads me to think it's a prefabricated flue is that there is no chimney breast and the visible part of the flue, on the roof, is a round section metal pipe something like the metal one on this page...

http://www.stovax.com/chimneys-flues/ch ... ves-fires/

The wall above the fire (which seems to be plasterboard as it's quite hollow when tapped) has developed a narrow vertical crack roughly in line with the centre of the fire, which runs from the ceiling down to the top of the mantlepiece, and may well go down behind it.

Additionally, the wall is bulging outwards slightly, almost as though the plasterboard has expanded and pushed out a little where this crack is.

This is only on the inside; there is no issue with the brickwork outside.

What I'm unclear about is how a prefabricated flue is built into the wall. Our previous house had a precast flue, which I know is simply built into the inner layer of blockwork, but I can't find any information which illustrates how a prefabricated flue fits into the wall. Does it run in the cavity between the inner blockwork and outer brickwork (seems unlikely to me as the flue diameter on the roof seems larger than a typical cavity width), but if not, how does it fit, and is there likely to be a continuous inner course of blockwork in front of the flue?

I'm thinking of removing a little vertical strip of the plasterboard where it is cracked to see if I can remove the slight bulging, then refill the gap.

Not a job I'm relishing as the wall (all the walls, in fact) are covered in an anaglypta type paper and emulsioned, so it will entail a quite extensive redecoration. But I'm hesitant to do this as I'm not entirely clear how the wall behind this is likely to be constructed.

Any hints suggestions or pointers will be gratefully received!

TIA

Staffordian