Didn't know I had a cellar..issues

ckeen

I was recently informed that I had a cellar under my property. I knew my neighbour did, and in fact the developers turned there's into a living space. However whilst stripping back their flood damaged walls they found  a bricked up passage. We removed a few breeze blocks and discovered a small vaulted ceiling cellar under my property. Which was flooded. There is also a large active sewer pipe on brick pillars running through it. Not only did I know nothing about the cellar, all surveys taken do not flag up this sewer. I also have zero access to the cellar.
On top of that there are very clear signs of pumps being used in the past like sump pit and hp piping.
Where on earth do you think I stand?
I should say that this is an 1860s pub converted into four freehold dwellings back in 2016.
I'd like it dry for both my sake and the neighbour 
Thank you

Screaming_Flamingo

Wow. I have absolutely no idea where you stand, but just wanted to add a note of sympathy because it sounds weird, stressful, and also... a little fun? Perhaps there is a buried chest down there too filled with gold? 

But I would think first stop has to be the lawyer you used to buy it. It sounds like there are a hundred and one complexities here (access, responsibilities etc etc) and you will need a lawyer at some point, so it might as well be before you discover any problems. 

Hope it goes ok (and fingers crossed for the hidden treasure)