@Chrissie A lot of people report winter DDs being raised to cover winter monthly costs. It seems to be a new DD policy from SP (and other suppliers). Presumably the DD will be put down again in April. So it is probably not connected to your smart meters, the same would happen with conventional meters.
@Chrissie As another check have a look at your kWh readings from the smart meter. They should be showing the same monthly consumption as before. This will tell you if they are really at fault or if it is caused by SP’s DD policy. The only thing the smart meter will do is send more frequent updates, there is unlikely to be any change of kWh consumption compared with your previous meters.