The reply from @zapped is unrelated to smart meters, they’re referring to non-smart E7 meters.
The problem you have is that you have a 5-terminal meter, where there is a permanent live output as normal, plus a switched live that is controlled by the meter and live only during the off-peak time periods. Whilst this auxiliary output does appear to be in the smart meter specification, it seems that no one actually makes any smart meters that support it.
Your only option is to get a local electrician to remove the need for your switched-live timed circuit. This could be done by making it permanently live, and then using separate time switches on the equipment that needs it, presumably storage heaters and/or a hot water tank.
This type of meter controlled circuit isn’t used any more, or at least has become unpopular. It’s better and more flexible to have individually controlled appliances.