Fraud Investigator/lawyer and Possession Defence query

Possession - Section 8 and Section 21

Hello all,

Two questions:

1st we believe our tenants committted fraud/false statements with their references: believe gave false landlord reference  and false employment reference. Looking into raising a criminal investigation. My question is what kind of lawyer can look into references? Is It a fraud investigator who works with a lawyer? This person will need to access the full referencing from a guarded estate agent (they are arguably negligent) and approach the company who gave the employment reference for verification. Looking to build a case for when we have got possession of our flat back or even before as a stick to get them out.

2nd question related to above we used a specialist eviction possession company to check over S21 documents and submit possession order and money order when the tenants refused to leave on expry of S21. We provided witness statements that the S21 prescribed douments were in order. Everything should  have been in order. However when the hearing came up the tenants (aided by duty lawyer/solicitor) raised the  defence on the day that they had not received prescribed documents at the start of the tenancy (which they had). My question is should my legal guy present been able to defend that on the day. Seems a very common defence.

Yes we are aware that digging around for fraud and side investigations can impact the ongoing court possession route.

Many Thanks