The agency will assist the in-house PR teams on consumer and trade PR for Bravo, Living TV, Challenge? and Trouble.
This is the first time that a retained agency has been appointed to handle PR for all four channels, whose PR was previously carried out in-house with agencies assisting on a project basis.
The THC team is headed by co-founder James Herring, reporting to Flextech's marketing director Mary Beer.
The appointment of THC follows a three-month project carried out by the agency for Living TV earlier this year, which aimed to promote its Paranormal Zone season of programmes.
PR activity that the agency will be assisting on includes promoting the third series of US sitcom Will and Grace, which will be screened first in the UK on Living TV. The second series is currently being shown on Channel Four.
THC is also assisting on a campaign to promote Bravo celebrity documentary show Hellraisers' Handbook.
Other Flextech channels are UK Gold, UK Horizons, UK Style, UK drama and Play.