@kevin58, throughout your contract, you would have received your service at a discounted rate (relative to the headline rate, if you search the forum under "pricing" or read the legal link at the foot of the page). The service could never just suddenly stop at the end of the contracted period, as customers would be left with nothing, regardless of whether or not an upgrade was pending. The precise date for Full Fibre going live is difficult to pin down even in relatively straightforward situations, so there could always be a margin for error.
When you put in the order for Fibre, that did not amount to the same thing as cancelling your service. Eventually accounting would have been updated to reflect the upgrade.
When that fell through, the default charge for your service would apply unless you simply renewed the older type of package and started a new contract at a better rate.
It might have been possible to haggle at the time, as you were considerably inconvenienced by this, but this is now water under the bridge, as you have left.
As long as you have paid now for the 30 day notice period, you should be fine. It can be unwise to cancel the Direct Debit when leaving, as it can incur charges while final billing is unresolved and also makes it more complicated reclaiming any potential refund.
If you would like staff to look into this for you, please complete your community forum profile details with your Talktalk landline number, as these are still relevant for identifying your account. You need to go via your avatar/name; settings; Personal Information. SAVE CHANGES.
Staff reply during the week.
Gliwmaeden2, a fellow customer.
