You need to chase up extra energy to refund your balance. Your new supplier may be able to do this for you. Arrange for an appointment to obtain a reading from your meter so that your readings are up to date.
SP seem to have failed to properly communicate with their customers. The only justification for SP using MDS is for the latter to act in the role of contractors/agents providing SP with confirmed physical meter readings. MDS's role should be disclosed to SP's customers by SP. MDS could also be remunerated for delivering a card to absent customers prompting them to submit readings to SP. It does not seem appropriate for SP customers to provide readings to MDS although MDS could argue that this allows them to target customers who do not co-operate in ensuring that SP have credible meter reading records. IE MDS might argue that their services reduce SP's bad debts thus benefitting the majority of customers who pay for their energy in a timely manner.