Replacement of Back Boiler with ILFE Gas Fire from B&Q

Our existing back boiler has been removed from the living room fireplace and replaced with a conventional boiler that lives elsewhere.

The flue liner that was attached to it has been left in place and is hanging down into the fireplace recess. We know this needs to be removed as it has reached it's end of life and can't be reused.

The house was built c1910 and I am making the assumption that the brickwork will not be suitable to be left unlined. Looking in the loft I can see cracks in the mortar that mean the chimney is unlikely to be sound and suitable for use without any sort of liner.

From the fires manual it looks like the solution in this situation is

to install the fire inside a gas flue box which connects to a 125mm flexible flue liner. The gas fire is rated at 3.5kw and the manual states that a 125mm flue is suitable. No additional ventilation is required in the room for this fire according to the size of the room.

I have found what looks like a suitable box at:

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meets the dimensions required in the gas fire manual and would need to be connected to a new flexible liner which terminates to a GC1 terminal.

My intention is to install the flue myself (this is covered by building regulations which means I will need to notify building control.. and pay?). I will also install the fire surround myself ready for CORGI man to make the gas connection and drop the fire in place. I will also carry out the relevant smoke tests myself beforehand to make sure the flue draws correctly although I will be expecting CORGI man to do these tests again.

Questions (Ed Sirett are you out there??):

1) My research has also found that in some instances you can install a debris plate rather than a complete flue box? This would be easier and cheaper as I have rebuilt the fire place opening to the exact size of the gas fire. If I have to now install a flue box I am going to have to make the opening slightly bigger which will be a real PITA. In what circumstance is a debris plate acceptable rather than the complete flue box ? 2) This is difficult to explain but here goes.. The fire place back panel when fixed to the fire surround does not sit flush tightly against the wall. Although the surround itself is tight to the wall the back panel is slightly inset. Should this void be packed out with fireproof board or similar ? 3) Should the flue box protrude from the builders opening so that it sits against the back panel? This would solve q2.

Any further help with this is much appreciated. Should I face up and be leaving this to the pro's ?