Help identifying mystery pipe under stairs

Dec 12, 2022
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United Kingdom
Hi all - first post, thank you in advance for any guidance!

I have a ~1904 built terraced house in SW London. There is a damp patch on the entrance hallway wall just about where the stairs start.

Looking in the cupboard under the stairs, it seems the mains water enters under the hallway then appears up under the stairs. There look to be signs of a very slow leak somewhere in this area, so I'm resigned to probably needing to take the hallway floor up and get a plumber to replace a load of lead piping.

Where it gets interesting is...the additional mystery pipe. I've circled in yellow my mains water - you can see it's still lead coming in, then has been joined to a modern copper pipe.
However circled in red is another pipe that has been capped off (and seems to have some weird crystals growing on). I'm unclear whether this is likely to be an original gas pipe ( you can see a modern yellow one running to the left of it, or potentially another water pipe.

Ideally, given the hallway floor is going to have to come up to investigate, I would clean up anything else down there as well, but obviously if it's a gas pipe that's not something a plumber will touch.

Any thoughts on what the red-circled pipe is likely to be??



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