I think that is set out in the T&C which you agreed - basically line rental refund and should be no charge for broadband
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If this is because of a fault or delayed provision then they should base the compensation off your package price and the exact length the issue took to resolve.
If your issue is not resolved yet then don't accept any offer of compensation as this should be calculated once the problem is fixed. Bare in mind if it's a provision then they will refund from the original date you were supposed to go live not from when you placed the order.