Thanks
Upthehatters200 for your support

By the way, I was locked to the Sunrise network for the calls so I don't understand how I could have been charged a different rate.
After a few more e-mails between myself & Tesco support they have agreed to offer me a full credit for the calls. They have not explained why this was happening but it suggests to me that something their end is or was broken!
Anyway, if you are on the Tesco network I would suggest looking at your roaming charges carefully to make sure you are being correctly charged.