Sorry you are having these problems Grandad, just as well you are on the ball. I listened to a recording over the weekend on Radio 4 Moneybox when the exact same scam was being played out on a chap, fortunately he had recorded the call and it was this that was being played back to the listeners. Apart from him having the usual Indian sub continent accent

he was pulling the same refund scam and the guy being duped was not aware and was actually going along with the callers request to make certain changes to his PC. I could tell what was coming as I have often warned you all about these low-life scumbags, but unfortunately this guy went along with all his requests and it was only some sort of protection that he had on his PC that flagged up a warning that thankfully made him put the phone down on the scammer.
I have said this before and I will say it again,
"No Bank or organisation will contact you over the phone for bank details" the same goes for emails.
"You should end the call immediately and wait at least 10 minutes before phoning the bank or organisation yourself" this is because the scammers may be keeping the line open and you think you are ringing the official organisation when you are still connected to them.
"Never use a phone number given in these emails or phone calls" Always wait as said earlier and then use the phone number that you can verify as your bank or organisation.
"Never fall for someone telling you that a courier will collect/deliver cards or cash or refunds" it is just another con.
I know that you have done the right thing Grandad, but this is just a timely warning that these scumbags are very convincing, so don't get caught. In your case Grandad although Talk-Talk are denying any guilt, it is they who are ultimately responsible for you getting these emails and calls and I am sure many other scammers will also have your details to try it on with, so be watchful.