I believe this service only currently runs bi-hourly on Sundays, via Bolton.
The 2-hourly Sunday service was introduced in December 2008 - prior to this, mid-Cheshire line services did not operate north of Altrincham on Sundays, I believe as GMPTE did not provide funding.
GMPTE were funding Sunday Altrincham-Manchester services but in the early 1990s they suffered a funding shortfall as a result of funding Metrolink so decided to stop funding Sunday Altrincham-Manchester in 1992 and arranged for rail tickets to be accepted on Metrolink services on Sundays only.
In 2008 Northern decided it was financially viable to enhance Sunday services on the Mid-Cheshire line despite not getting any funding from TfGM or the Cheshire councils unlike the Atherton, Hazel Grove and Stoke lines. According to MCRUA Sunday passenger numbers increased by 400% as a result of a 2 hourly Chester-Stockport-Southport service being run instead of a 3 hourly Chester-Altrincham service.