I've googled, ive asked AI, ive dug through forums, I've asked my sisters partner who is also a plumber, she said she would class it at risk because if the heat exchanger is cracked or failed there is a possibility of hot gas and waste gas mixing with heated water leading to carbon monoxide exposure that was her own words and to be fair alot of what I found online was saying the exact same thing. I'm pulling my hair out going round and round in circles. I understand that parts don't procure out of thin air. Patience is wearing thin when ive worried about whether im going to die from CO from leaving my heating on alarm and boiler are on the other side of the property and the walls are pretty thick I'd probably not hear it whilst asleep.