Flue sealing

Delurking to ask for help with sealing a flue.
Have just removed/dismantled the ancient Aga that was here when we moved in 8 months ago (originally solid fuel but diy converted to oil - you can watch the tank gauge falling when it runs) and am left with a stub of 100mm dia flexible flue liner protruding from the kitchen wall 900mm above the floor. How do I go about blocking/sealing this?
Have read lots of warnings about needing vents/airbricks but would really rather not have anything in the middle of the wall - the intention is to fit a radiator there when the CH is redone. The liner goes into the brick chimney stack that's in the middle of the bungalow - there's a built in woodburner in the lounge on the other side of the stack and a single chimney pot visible on top on the woodburner side (yes I know I need to climb up there and check what's happening on top but I hate ladders and haven't got a roof ladder).
Can you point me in the right direction?
Thanks, David
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