As has been said, PV immersion diverters are readily available - although it maybe makes more sense to run the heat pump to warm the tank than use an immersion - presumably the heat pump will run anyway if there is demand, so really you'd be asking the heat pump to run when there wasn't demand (but 'free' electricity available) and put the excess heat  in the tank ... which might suggest a need for a larger tank that you might otherwise have?

The idea of "free" PV electricity might be going away though - the "deemed 50% export" is going to disappear as smart meters measure actual exports and suppliers seemed to be obliged to switch to paying on a metered basis even for existing FIT contracts, so using excess generation locally will start to cost actual money.  Granted the per unit price is still smaller than importing - but the difference is declining - my supplier is already offering 10p/kWh in some circumstances - so getting to be in the same ball park as importing gas, especially when you factor in the cost of the extra equipment.

   - Andy.

As has been said, PV immersion diverters are readily available - although it maybe makes more sense to run the heat pump to warm the tank than use an immersion - presumably the heat pump will run anyway if there is demand, so really you'd be asking the heat pump to run when there wasn't demand (but 'free' electricity available) and put the excess heat  in the tank ... which might suggest a need for a larger tank that you might otherwise have?

The idea of "free" PV electricity might be going away though - the "deemed 50% export" is going to disappear as smart meters measure actual exports and suppliers seemed to be obliged to switch to paying on a metered basis even for existing FIT contracts, so using excess generation locally will start to cost actual money.  Granted the per unit price is still smaller than importing - but the difference is declining - my supplier is already offering 10p/kWh in some circumstances - so getting to be in the same ball park as importing gas, especially when you factor in the cost of the extra equipment.

   - Andy.