You need to look at the deeds as the loft is probably common to all the units.

Is there a property maintenance for the building

Its' usually brought up in conveyancing, but yes, you need to look at flying freeholds and who owns the property freehold.

Don't think you could get them removed, you'd need a Solicitor for that question, but I would certainly say they are responsible for the upkeep and any damage, much the same way your insurance should cover anyone below you if you have a leak.

I agree. Very unlikely you can get the pipes and tanks removed unless the neighbour agrees. Covenants/wayleaves normally have some value. In this case you have to ask yourself what the value of that space to the neighbour. Even if the neighbour agrees to move the tanks for nothing you would probably have to pay the legal costs of getting the covenant removed.

The neighbour would normally be responsible for maintenance and the consequences of any leaks. Can be difficult to enforce if they simply refuse to pay. You would probably have to go to the small claims court eventually. If that's likely you need to plan ahead -  if you have to pay out to get a leak fixed try and get multiple quotes, keep all bills and receipts etc.

If you have frequent problems you might have a case for statutory nuisance. If you write to them pointing out that their failure to maintain the pipes may lead to water damage to your property, and they fail to take action, then I believe that may allow the court to award damages for negligence as well as the cost of repairs.