Are you sure you own the loft?  Just because it's above your flat doesn't necessarily mean you own it. In many cases the flats in the block have a joint share in the loft and other "common" areas such as the roof, garden or entrance hall. Who pays for maintenance of the roof tiles?

If you really do own the roof/loft then that might not be sufficient to get the tank removed... It's possible the neighbour has an easement in the deeds or they may have acquired a right (through passage of time) that allows them to put/leave a water tank in your loft. Just because they don't need it anymore wouldn't give you an automatic right to remove it. Check the deeds. Find out on what legal basis the tank is there.

If they don't have a legal right to put a tank in the loft then you could ask them to remove it.

However if I was your neighbour I would check the deeds. If they give me a right to put a tank in your loft I would only let you remove it if you agreed to pay and pay to have the deeds changed. If there was nothing in the deeds I would claim I had a right to leave a tank in your loft based on the passage of time. I would reluctantly let you remove it if you pay Sorry.

Hello,

I own the top floor flat (+ non converted loft) of an old Victorian house in London. The house has 5 flats and each flat owns a share (1/5th) of the freehold.
The water tank of my downstairs neighbours is located in my loft. The flat was recently sold to and converted by a property investment company. Following the buildings works, the water now comes directly into their flat from the mains, which means that the old tank in the roof is redundant.
Their team of builders mentioned that they could remove the old water tank and dispose of it. They will come to take a look at it soon.
What is unclear to me is who is supposed to pay for the removal of the tank. It could be quite an expensive job, especially as there could be asbestos involved.
Whilst in use, I couldn't have disposed of it if I had wanted to. But now that they have made it redundant, can I have them remove it and ask them to pay for it?
What if I did not have it done now (I understand that it can be quite a messy job) but only in a few years time when I want to convert the loft? Could I ask the (potentially new) owners to pay for the removal?

Thanks in advance,
Cecile