Reno works damaged council flat. Council is unreachable to agree on repairs. How to proceed?

TLDR: Building works on our flat caused hairline cracks in our neighbour’s plaster. Neighbour insists remediation works have to go through their landlord - the council. The council is unresponsive. What can we do to get this resolved?

We’ve had building works done on our flat which have cause cracks in the plaster of the downstairs flat. We have a contract with our builders that ensures they remediate any damage to surrounding properties. We want to fix the cracks and our contractor is keen to carry out the fixes per the contract.

Neighbour is a council tenant, denied access for fixes and instead raised a complaint to the council (their landlord). Council went round, took pictures, and requested we carry out repair works.

To carry out repair works, they specifically requested for our builders to contact a particular member of the council to 1) discuss remediation plans and 2) schedule a date for the works to be carried out.

Problem is, this person is unreachable. We tried contacting them upwards of 20 times over the past week. Email communication typically takes 2-6 weeks to be responded to, and the last email very firmly requested for our contractors to get in touch on a given phone number.

How can we reasonably go forward on this? We’ve been trying to arrange repairs since the beginning of January. Our works are nearing completion yet this issue of repairs (which we all want to carry out) is still outstanding, with no sign of progress.