"a cost of £600m – which it planned to recoup by hiking up bills"
IIRC, they were proposing to put up wholesale bills by £7 per month (presumably forever).
By my calculation, across 20m households, that works out at £1.68bn per year. Nice little earner??!
Adding £2.50 per month per line, for one year only, would cover the £600m outright.