You've got them by the short and curleys because they've admitted it's been installed incorrectly. DNOs are a law unto themselves and just do what they want.
No good mentioning BS7671 to them it's out of scope for supply, they work to ESQCR. It's unlucky for them that the wall is on top of their cable but that's their problem not yours. Keep on and on at them to get it sorted and every time you speak to them mention that you think it's dangerous. If you report something that's dangerous they have to come out and check, so every week give them a call and tell them you think something has further damaged the cable. If that fails get onto OFGEM who regulates them.
Link to ESQCR https://www.legislation.gov.uk/uksi/2002/2665/contents/made
No good mentioning BS7671 to them it's out of scope for supply, they work to ESQCR. It's unlucky for them that the wall is on top of their cable but that's their problem not yours. Keep on and on at them to get it sorted and every time you speak to them mention that you think it's dangerous. If you report something that's dangerous they have to come out and check, so every week give them a call and tell them you think something has further damaged the cable. If that fails get onto OFGEM who regulates them.
Link to ESQCR https://www.legislation.gov.uk/uksi/2002/2665/contents/made