It certainly varies from situation to situation, and with a heat pump you are a very heavy electricity user.
I think it's unlikely that extra batteries will pay for themselves: in summer you won't be using the heat pump much, and in winter most days you won't generate enough electricity to fill up 15kWh of battery in addition to your normal daily usage, even with your large array - unless you charge from grid.
There are two tariffs from Octopus that you should look at. "Cosy Octopus" is especially for heat pump owners; it has two low-rate periods, 4am-7am and 1pm-4pm, which you can use for heating your house and/or charging batteries. You could combine this with one of their outgoing tariffs for exporting any surplus, and your existing battery will probably be enough to cover the gaps between the low-rate periods.
"Flux" is a new tariff from Octopus for PV+battery owners, which only has one low-rate period (2am-5am) but pays a much higher export rate. I suspect that Cosy will suit you better, but check them both out - and in any case you can switch between tariffs without penalty, as often as you like.
If you do become a new customer to Octopus, please quote my referral code "plum-fish-550" when you sign up, and then you and I will get a £50 credit each :-)
