Tenant refuses to allow access

Maintenance

A tenant in a flat we own has repeatedly refused access to us and tradesmen, starting in March 2020 with covid restrictions. Since then we have tried on numerous attempts to arrange an EICHR, gas checks, a builder to fix a ventilation problem. As a result of the latter, mould is now affecting the property. His only concession is for us to drop off mould killer, but refuses us access to inspect. The tenant has also cancelled at the last minute arranged tradesmen visits, incurring us extra costs.
I would be grateful for advice as at the moment we are considering giving up and selling it!