All time cost query

Does the all time cost take account of the different cost charged by a previous electricity provider or just take the currently used cost and work it back to day zero?

Each session tracks the cost as it happens using the tariff at the time, and then the total adds all of those sessions up. So it will include different tariffs from the past. Note that time-of-use pricing was only added as a capability midway through last year, so really old sessions will have been calculated with a flat price.