I slide a cutting mat underneath, then put the pins in from above without hands underneath. But what do I know -still putting off re-doing the quilt that got creases in the back when I sandwiched it recently!
I visited the local painter and decorator shop to ask about the blue painter's tape that was recommended to me here. I think he was a bit surprised to be helping someone with their sewing, but confirmed that this tape is available in the UK. He didn't have it in stock though - but found me some that was designed to go on wallpaper and delicate surfaces, so I'm hoping that will do the trick.