When twin brothers Riccardo and Simone Bertagna bought a share of a three-bedroom flat in Seven Sisters, north London, they told me they felt "proud". "It felt good. I felt I achieved something," says 35‑year‑old Riccardo. They bought a 25% stake in the property, which had a total value of £440,000, through a housing association. It meant paying a mortgage, rent on 75% of the property, and a service charge of around £90 a month. The brothers saw it as a good investment and "more stable" than renting in the capital. It also worked out at £50 a month cheaper than when they were tenants. But a few years ago the flat was devastated by an external leak. Rising service costs and disputes about who was responsible have left the brothers feeling "trapped" in an "unsellable" home. They say that despite the leaks, a broken lift, "dirty" communal areas and "no visible improvement to the property or complex", the monthly service charge has continued to rise to just under £300.…