How do I stop endless water coming through the ceiling of my rental property?

I have a second floor flat where there were frequent reports of water leaks into the flat below. Upon invasive investigation it was found that the copper pipework had been set into the concrete floor of the flat, without any protective lagging, so that over time the acids in the concrete gradually dissolve the copper causing "pinholes" in the pipework.

This is clearly a construction defect as it is built in to the fabric of the structure.

The freeholder denied responsibility, the block management company denied responsibility, and of course I as the leaseholder of the flat also denied responsibility (as it was nothing to do with my flat itself, it was a construction fault, and the same issues were happening with other parts of the building).

After 2 - 3 years of arguing with the block management company, and temporary repairs via the block insurance policy (via the block management company), it was eventually agreed that the block management company would pay for the re-piping of the water system so that all pipework is above the flat's floor level (so would no longer be set in corrosive concrete, and pipes could be accessed without digging channels and holes into the concrete floor of the flat). - There's been no problems or leaks reported since that date (about 5 years ago).

Hi Helen
You said you are renting out your flat in a property of flats. Your flat is owned by you as a tenant with a lease.
If that’s correct then there is a freeholder who owns the property who has repair obligations.
The MA should report these leaks causing damage to the structure of the property to the Freeholder who can claim on the block insurance and get a survey to identify the problem and organise repairs

Helen, do not fall for a boiler quote of £6000, that is a rip off so back off and seek local small time Gas Safe engineers who would replace and service your boiler at less than 3 grand, you can get a decent boiler for about a grand and another grand for taking out old boiler and replacing it with similar size boiler, so around £2,500 at the most.
For the leak, your tenant needs to call you when it is actually leaking and not after it has dried up, that is the time when one would invite an upstairs occupier to come down to see for themselves, that way you may be invited in return to go upstairs and correlate exactly what is situated above that leak, i.e. is it a bathroom, a kitchen or whatever and should there be a reasonable possibility for a leak to develop, once a year seems odd, which could be that occupiers above may have spilled a bucket of water accidentally or perform some deep cleaning task such as mopping with too much water, or accidentally spilling a bucket of water.

Sounds like you are a leaseholder in a block of flats with a separate freeholder and managing agent. Consult your lease, there can be a bit of variance on the terms.
In general, with newer leases the freeholder owns the structure and main supply and drainage. This kind of leak between flats is affecting the structure. The freeholder can enforce on the offending leaseholder and if necessary claim on the block insurance.

Some points you may find helpful:
1. With flats, the lease is a good place to start. You may find there is reference to upkeep and condition of property and the onus placed on flat owners in this regard; are they negligent here and breaking the terms of their lease? You may have recourse if they are, including with their lender.

2. Are the occupiers the flat owners? Focus communication on the owners.

3. Suggest to the above flat owner you would like to send a competent plumber to investigate whether there is an issue at the property e.g. poorly sealed bath etc. You might pay for this, but you will have confidence knowing what the issue actually is and can then act on this accordingly.

4. Why are you paying for remedial work when you are not at fault? This is ordinarily covered by building insurance for the block. The managing agent should advise on this. Also, repeated insurance claims because of this issue will likely increase the block's premium which is something you could advise other owners of - may give you extra leverage over the owner upstairs to get the situation properly rectified.