My question involves landlord-tenant law in England, UK.

I wonder if anyone can help me.

When we first took out leases on the house we rent it was done through an estate agency.
About 2 months into the lease the landlord contacted us to ask if he could come round and see us as he needed to get the house surveyed. We agreed to see him and he told us that when the house is surveyed he would need to be there as he is getting it re-mortgaged as if he is still living there rather than buy-to-let because it is cheaper. At the time we thought nothing of it.
The landlord is a self-employed accountant and had the property we live in as a business premises when he still "lived" there.
Since the re-mortaging it has played on my mind somewhat, as we still get an awful lot of letters for him for the home and business. He gave us his new address at the beginning of the tenancy so I used to re-direct all his mail there but since there has been no let-up in the amount we receive for him I have started returning it to sender. A couple of weeks ago we received his car tax reminder letter, which now makes me think he is making out to everybody that he still lives here.

First of all, is this legal, and secondly are there any implications on us if it all comes to light?