I don't think it needs to be serviced by Mg to keep the warranty, as long as its been serviced by a reputable company it would keep the warranty.
It has to be an authorised MG garage.
Warranty Terms and Conditions
MG Motor UK reserves the right to make changes in content, description or terms of the warranty policy if it’s
deemed necessary by legislation or business objectives and agree to give MG Dealers notice of any changes.
MG Warranty conditions require that:
• All claims are notified to, and carried out by, an Authorised MG Repairer during the Warranty period.
• Warranty paint rectification must be carried out by an Authorised MG Bodyshop in line with the MG Body
Repair Manual.
• All repairs, damage rectification or fitting of replacement parts and accessories are carried out by an
Authorised MG Repairer and in accordance with MG instructions.
• The vehicle has not been altered from the original specification, where said alterations maybe found to be
the cause of any subsequent component failure.
• The body panels are inspected annually throughout the warranty period by an authorised MG Repairer
(relevant paperwork completed and endorsed), and any rectification work found necessary is carried out in
accordance with MG’s recommendations.
• The vehicle has not suffered from neglect, improper repair, or improper use, and has been properly
cleaned, maintained and serviced in accordance with MG’s recommendations.
• Each main service is carried out within 1000 miles / 1609 kilometers or 28 days of the recommended
mileage interval or service anniversary date as shown on the Service Interval Plan (within the service
portfolio) and that the appropriate service record page is stamped and endorsed by the servicing agent.
Whilst the warranty will not be invalidated if a non-authorised repairer carries out this work, Warranty
faults resulting from work carried out by a non-authorised repairer may not be covered under the MG
Warranty.
• Note: To qualify for the MGZS 4th, 5th, 6th & 7th year warranty, the vehicle must have a full and accurate
service history, in line with MG Motor UK’s recommended servicing plan, using genuine parts and fluids to
the correct specifications