Fan flued appliance should usually be 300mm from openable window. Unless stated otherwise in manafactures instructions.
There is an issue here though of the flue being too close to combustible material (window frame).
Also your open flue appliance needs ‘purpose provided’ ventilation and your chimney flue needs inspecting.
Boiler would be classed as ‘At Risk’ and should be turned off.
The fire is ‘Immediately Dangerous’ and should be disconnected.
Also to whoever installed open flue appliance with proper ventilation, this is RIDDOR reportable.
I’ve not turned it back on, and won’t. It’s just frustrating as I’ve been suffering headaches since moving in. Wheather they’re connected or not, no idea. Just seems like everyone’s trying to cover their bum, and all I’m asking for is a safe property to live in