Something is dead in our chimney

[PuraVida]

And in the fireplace is a back boiler and ghastly gas fire

I've been sniffing the smell of death for a few days now. It's reached rhe point of overwhelmingness

There's nothing I can do is there :( ?????

You may need to remove the fire/back boiler and get the "dead" thing out, otherwise the smell will get worse and the room will fill with bluebottle flies. My friend had this happen to her and this is what they eventually had to do as the smell became awful. See if you can get a cowl put on the chimney pot on the roof.

I really hate to be the bearer of bad news but the smell will worsen and even when it dies down it will linger for ages. And depending on what is dead and how big it is you will have at least a week or so of constant large flies. If at all possible get it out.