Block off/register/closure plate - cold air around the edges

When we bought this house, our stove had a plate above it around the flue that was, frankly, dangerous - full of combustibles and just not fit for purpose. We have since got someone out to fit, what I think is referred to as, a closure plate. We are using the original chimney stack but there’s a liner connected to the stove. What we’re finding is freezing cold air coming around the edges of the closure/block off plate when the log burner isn’t in use- this gets worse when it’s colder outside.

I’ve had a few chimney folks out for sweeping / fixing it but none of them have recommended anything for this. I’m keen to stop the draft around it- any tips on how to do this safely? I have some fire cement but don’t want to yolo into something if there’s an easier/bigger problem lurking.

Ideas welcome!

When we bought this house, our stove had a plate above it around the flue that was, frankly, dangerous - full of combustibles and just not fit for purpose. We have since got someone out to fit, what I think is referred to as, a closure plate. We are using the original chimney stack but there’s a liner connected to the stove. What we’re finding is freezing cold air coming around the edges of the closure/block off plate when the log burner isn’t in use- this gets worse when it’s colder outside.

I’ve had a few chimney folks out for sweeping / fixing it but none of them have recommended anything for this. I’m keen to stop the draft around it- any tips on how to do this safely? I have some fire cement but don’t want to yolo into something if there’s an easier/bigger problem lurking.

Ideas welcome!