Pete- why would you be after a MATS reference? Airmen are not accountable to a MATS. If you are checking them, surely your reference would be the AIP or similar?
The requirement to comply with a clearance is stated there (AIP), and is not cancelled by a frequency change. The communication aspect of the frequency change is to re-state your cleared level and callsign only, unless instructed to advise anything else. Your compliance with all previous instructions remains unchanged. AIP refers.