billed for full contract?

Unfortunately @Gazza Coll, by getting and using a PAC you’ve cancelled the contract, not the order.

Technically there’s a difference between cancelling your contract, and cancelling your order within its cooling off period, which iD systems don’t seem to handle very well @Gazza Coll.

If you don’t mind waiting a day or two, someone from iD may read your post and reply. 
Alternatively, I’d suggest using Facebook Messenger or a Twitter DM, to leave iD customer service an out-of-hours message about this issue. 

The agents work until 8pm on weekdays (6pm at weekends).

Not sure what you mean by a cancellation number @Gazza Coll? 
If you cancel an order, you’ll be given a returns number by iD.

If you’ve cancelled your ORDER, your line rental is converted to PAYG, on the iD SIM card received with your order. If you requested your PAC, and gave this PAC to a new service provider, before your line rental was converted to PAYG, you haven’t cancelled your ORDER. There’ll be an early termination charge.

Hopefully iD will sort it out, but this might not happen quickly if you communicate with iD by posting in this forum. The iD staff only read forum posts once a day, sometimes less frequently.