can shops add a surcharge when using your switch card to pay ?

Yes, they can, they get charged by the banks for taking switch payments and they can pass the charge on to you.

Shops can charge what ever they like for using any type of card payment. However the charge on the shop for using the switch is quite minimal and they get their money straight away as it is a direct electronic transfer. There is no effort to the shop. It is up to us as consumers to tell the shop that the product that you are buying can stay on the shelf unless they waive the additional charge. Be prepared to walk out and do not purchase from them. I have made this threat and the answer given was, "On this occasion we will not make the charge". If we all refuse these "extras" then they will stop trying it on.

These days a lot of businesses are seeking ways to screw the consumer out of more money without the need to make an effort or actually sell anything.

yes they can but there has to be a notice at the till saying how much it will be and for what reason

Yes, they are charged for doing it and as such can charge you