Reporting ex husband for mortgage fraud

Octopustentacles

My ex husband has a house solely in his name. Before we split up, my ex told me the house that he is renting out is under a residential mortgage. He has never lived in that house, nor has he any intention of living there. 
I am also pretty sure that he is not disclosing rental income so this comes under tax evasion 
I checked with the land registry and the house is still in his name and I know the name of the mortgage company.
What other information would I need? Who would I report to? Can I do this all anonymously? 
What would be the implications should he get caught?

ACG

You would report the mortgage issue to the lender, they would decide what to do. 
The tax issue would be declared to HMRC, they would decide what to do. 

The lender is unlikely to report it to the police, they may call in the loan and give him a month or so to get a BTL mortgage, they may put him on the national hunter system. HMRC would probably fine him, I doubt they would put him behind bars as he will just plead ignorance and pay any tax due with the fines. 

As far as I am aware it can be done anonymously but would it take him long to work out who did it?