Help with a Section 21

Possession - Section 8 and Section 21

I have a tenant who lives above my son’s shop. He owns the shop I own the three-bedroom flat. I acquired a tenant through the local council. The council did all the paperwork and guaranteed the deposit, they checked the flat and signed it off.
The tenant is not the easiest person to work with, always voicing tenant rights. She can be difficult at times.
I wanted to retire in July 2020 so issued her with a section 21, which I did myself. Unfortunately, there was a slight problem with dates, regarding a gas certificate. I took her to court and the judge ruled in her favor.
Can anyone point me in the right direction to find either a landlord’s group, reasonable price solicitor, or other organisation to assist me with the new 21.
I’m located on the South Coast in the Brighton area.
Many thanks.