What are the regulations in the UK with regard to point of demarcation and what the telco has to support in cases like this?
Here in the US, the telco would be required to provide correctly-functioning service at their point of demarcation into the premises, with no Customer Provided Equipment (CPE). So, that would mean that the three possible voltages on the line be within specification: the on-hook ~48VDC, the off-hook ~12VDC, and the ring voltage (varies depending on load and loop length, etc, but in the ~70-100VAC range). The telco should at least test and confirm that, with no customer-provided equipment attached to the line. They should be able to detect an intermittent short or high-impedance condition.