Tank in loft vs back boiler

[Mrs Pingu]

Some of you may recall from witterings passim that I have a back boiler in the fireplace that I want to block up.
I'm hoping to avoid doing anything with the BB at all, just get the concrete fire surround and hearth removed and the opening boarded over, but I thought I'd do some investigation in case the back boiler *had to* come out for whoever reason.

I've had a feeling there was a water tank in the loft, so today I investigated it. Under the lagging was a plastic 4 gallon tank that said it was for expansion or storage and proved to be completely dry. 
So given there's no other obvious big tanky looking lumps anywhere, do you reckon I'm safe to assume the back boiler is not connected to anything that is likely to be full of tons of water?

[Mrs Pingu]

Speaking of which I had a look at the photos I took when I went under the floor, this is the gas pipe that I need to get terminated below floor level
2022-01-29_01-29-39 by The Pingus, on Flickr

I assume there's a constructional hearth there but it looks a bit less constructed...
2022-01-29_01-29-53 by The Pingus, on Flickr

Also fireplace, back boiler and offending gas pipe
2021-10-17_02-32-12 by The Pingus, on Flickr

[canny colin]

I do hope the overflow tank was not for , an uncontrollable heat source such as an open fire or multifuel stove . By law and common sense , it should be galvanised steel or copper .   I will leave it up to your imagination what would happen , if the system over heated . The plastic tank melted and poured it content over the ceiling .   
I would double check the tank is empty  , try TAPING the tank with a hammer "gently" and tracing the sound up the system to the tank . There should be a drain off valve on  one side of the cheeks, unless the pipework has been removed  . I can tell by the sound the tank and pipes make if the system is drained , but its hard to explain . Worth taken time over , you don't want to come home to a leak because the pipes or tank have bust in the winter. It will very cold in that sealed up breast . I would always recommend fitting an air vent in the "built up opening" and in to the leeward  side of the chimney ,to stop penetrating damp   

[Mrs Pingu]

Yeah I thought plastic was odd, but maybe it's been replaced in the past for a tank for an immersion or a system boiler?

I put my hand in the tank all the way to the bottom, it's dry. Oh, you mean the tank in the fireplace?

[Feanor]

Is there any reason you can't drill the old back boiler just to be sure?

[Mrs Pingu]

Only not wanting to be the little (old) girl with her finger in the dyke if it turns out to be unexpectedly full.

Maybe I should try pointing the thermometer at it.

I have read comments about some people who have disconnected such things deliberately drilling a hole in the boiler tank to prevent steamy explosions.

[Feanor]

Well, the issue that the drilling is meant to mitigate is the one we had at our first flat.

The back-boiler is no longer in use and has been disconnected. But where has it been disconnected?
Shortly after we moved in, we set a fire in the back room where the back boiler was.
An hour later, there was a great hissing and spitting from the 'press' (cupboard set into the wall) beside the fireplace.
The pipes from the back boiler had simply been cut here in this cupboard when it had been de-commissioned.
The boiler itself still held it's volume of water, which was now making a bid for freedom!
So we damped the fire down, and the following day I drilled it and drained it.

In your case, I can't see where there's any ongoing source of water, based on your loft investimigations.
Can you even see where the pipes from the back boiler emerge? I imagine they are buried deep in the walls.

I'd be happy to:
1) drill it; and
b) cover it up however you want.

The usual disclaimer applies, and my tame lawyer has already told me 'On your own head, mate!'.

[Mrs Pingu]

Lol, that is a great story.
No, I can't see there is anywhere else for anything significant to be hiding either, apart from the pipework lurking in the guts of the chimney.