Property damage discovered midway through tenancy

Maintenance

Tenant reported that a window lock in the property kitchen was jammed during hot weather, so sent contractor to inspect and fix problem within a few days. The contractor also noticed the kitchen sink tap was leaking and had been leaking for some time as work surface is solid wood and that area had turned completely black with mould. The contractor, having decorated and prepared the property for let, asked the tenant if he had reported the leak and when did it start, to which the tenant confirmed he had not reported the problem yet and could not remember how long it had been leaking. The contractor took photos of the damage and reported it immediately. In response we authorised him to replace the leaking tap and he also incurred cost sanding the immediate area to get rid of mould, which is a health hazard. Contractor charged an extra £50 for this job and confirmed as worktop solid wood, he really needs to sand the whole lot down, seal and polish to restore to original appearance. Had the tenant contacted us immediately when the leak started there clearly would not be any damage. Their tenancy runs for a further year. Check in and checkout inventory has and will be done, and deposit held. This damage/cost of worktop restoration will have to be taken out of their deposit, as not normal wear and tear after 1 year. Should this and therefore the cost be confirmed/memorialised now for future reference? If so how shall we do this?