Just in case anyone else ends up wondering about experience of BG / Hive Freecharge and i4, I successfully carried out my first charge overnight. Basically you need to disable the i4 schedule and just rely on the Hive apps own schedule. Basically BG/Hive decides when it is going to charge and at what rate - you just tell it when you want the car to be charged by. The hours of charging were around 1pm for 50 mins and then some time late at night but reached the 80% limit set on the i4 at around 1am. I didn't charge by very much as just wanted to know if it works. The next step is to see if i do indeed get credited the full kwH amount on my next electricity bill - I can see how much consumption is due to be credited back in the Hive app.
If you do need to take control there are huge buttons on the app to "override" - it's almost a challenge to not press them as I am sure BG would rather you overrode all the time.

As a result of Freecharge i have stuck with the standard tarriff rather than the EV tarriff as no point in increasing my daytime costs if the EV charging is free anyway.
Will see how it goes in the mean time.
Backup plan if, for whatever reason it doesn't charge to set capacity consistently is to go with BG EV rate or switch to Octopus - gift horses and all that .

 

Octopus EV rate is better I believe (7.5p/kWh), and the ‘standard EV’ window is longer too (23:30 to 05:30) - or it was. I ditched BG for Octopus in March. BG‘s rate was around 10p/kWh at that time and the bit I didn’t get was they upped the unit price of gas on their EV rate!!

 

Let's see if "Freecharge" really is free - it should be as I've already paid for the Hive charger and basically the plan is reimbursing against it. I will jump to Octopus if it doesn't pan at as free or effective.