Nectar and gift vouchers

Hi
I have managed to save lots of Nectar points, even though I rarely use Sainsburys directly. I want to buy stuff elsewhere. I know I can spend them at Argos, but they arent always the cheapest place for what I want (Lego Technics).
Can I use my point to buy gift cards at the till, such as Amazon?
Customer services in the shop yesterday said no, said as the Nectar double up points were on, it excluded gift cards. I am not bothered about the double up, I just want an Amazon voucher! Other online searches people have said yes it is possible, just scan at the till and pay.
So before I try again in a different store, can someone say if it is possible or not. Or suggest any other place I can spend Nectar points to get Lego Technic!!
Thank you!!