Similar to you, original FIT, 3.7kWh PV, ASHP, PV diverter but no electric vehicle, yet.
My thoughts were
1. Leave existing PV as is and top up batteries and ASHP on a cheap overnight rate.
2. Use existing 3.7kWh PV to top up batteries when grid is down.
3. Add separate PV system to increase input.
Dim question, not sure how this works, you have a tariff which enables you to buy and sell units at different price points because you can use the batteries.