If you ask now for the PAC, they will at your request immediately bill any remaining line rental (so your remaining month), then as long as you use the PAC code before a new month (out of your contract) starts then you will only be charged for any call charges etc. you have made.
It works out the same as waiting since you will pay the same. If you wait till the end of the contract then it is likely you will be paying a week or so extra out of your contract (pro-rata).
great, thanks for the info.