correct form required for evection

Rent Arrears

The building comprises of a self contained shop on the ground floor.
with 2 flats on the first floor. I am the landlord, and live in one flat, the other is let on a assured short term tenancy agreement.
the tenant refuses to leave and is in arrears with his rent (DHSS)
His solicitor has told me that I can not use a section 21 form as it is not the correct form for this type of property, due to the fact that I live on the premises and the property is part commercial.
I am to say the least confused/frustrated. I have been letting this flat for 30 years with no problems!
HELP.