Companies reviews in the Buying and selling of own real estate in the UK

The worst company ever in the uk market. Unprofessional, unable to do their job and impossible to contact them. I’m Andrea barzaghi, and I’m the landlord of a flat they are managing in London. I’m trying to sell my property but it’s now more than 1 month I’m trying to have a document they are unable to share. The sale of our property is now at serious risk for this missing document and they are not replying to the emails (not only mine, but also my solicitor official emails and calls). The document we are unable to obtain is the completed Landlord’s certificate (from page 3) but seems all the companies involved on this are not able to produce it. The reference company acting in name of the builder is DR4 Developments LLP, part of Vistry group. Our solicitor is chasing both since 1 months but we are still waiting a single answer. I have chased them as well but I never received a single replay. Are this 2 companies still on a market or are they a ghost societies? How is it possible they are not replaying to the emails on something is their duty by law? Also, how comes they were able to give us a certificate but missing of some fundamental parts (the proof)? Does this means the builder and their associated management companies did something illegal? I literally bagged their help but nothing comes to us since weeks, putting our sale at high risk. I’m simply shocked about what is going on and the impossibility to talk with them and have a document they need to provide us.