So looks like it's almost 8hrs at the cheaper rate given this is what they've offered as a time scale for tonight.
Originally it was showing 00.30 as a start time it now looks like from 22.30 the cheaper tariff will run from
Hopefully I'll have some more info/update on it tomorrow/Weds.
I'd be interested to know if anyone else has gone down this route and is getting the same or can share their experience