The standard reply will most likely be that it will be 4 years or 60,000 miles, whichever comes first, This is a standard across all manufacturers' service guidelines. It might be worth checking with an independent garage to see their pricing, using parts with the same specifications as the OEM ones.
The wording in the Warranty statement is as follows:
"Each main service is carried out within 1,500 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 is communicated to the DSR Portal by the servicing dealer. 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-authorized repairer may not be covered under the MG Warranty. An itemised service invoice must be obtained."
"Note: To qualify for the MG 4th, 5th, 6th & 7th year warranty, the vehicle must have a full and accurate service history including a VAT invoice, in line with MG Motor Recommended servicing plan, using Original Equipment Specification (OE) parts and fluids.• If a defect arises from non-compliance of manufacturer-recommended service intervals and manufacturer recommendations for care and maintenance, the Authorised MG Repairer reserves the right to reject any subsequent claims.• MG-approved parts are used for repairs effected under warranty, except as an emergency repair, which must be made good within 14 days using components approved by MG."