1. In my experience, letting agencies don't have the first clue about plumbing, or very much else for that matter. So often don't have the knowledge to select the right people for any given job. This from experience as a landlord and plumber.
2. If you can, avoid anyone recommended by agents, and insist they use people you trust. Get the work billed to you directly if you can to avoid the agency uplift and possibly the addition of VAT.
3. That flexible hose:
3a. Is unlikely to be rated for constant hot temperatures if it is part of the heating circuit, although it doesn't look as if it is.
3b. Will restrict the flow .
3c. May well have left the copper at each end unsupported.
4. If the floor overlay of chipboard was previously wet then:
4a. The wet came from somewhere.
4b. Copper in contact with concrete or plaster, particularly if the contact material gets wet, can develop pin holes. So I wouldn't dismiss the claims of multiple leaks, although it would have been sensible for the "plumbers" to have kept, or at least photographed, the evidence.
4c. One could envisage the situation where a leaking waste soaked the floor, wet the concrete / plaster, and caused pin holing of the copper.