It was all systems go this morning and I started to paint that big canvas. Unbelievably the paint (I use professional acrylics) separated like I was using waxed paper. So I tried undiluted paint and it happened again. So phoned the supplier and it seems I got a faulty batch of Winsor & Newton's canvases.
I was just desperate to start and didn't want to send them back if I could help it. So I gessoed it and hooray the paint went on beautifully. So now it's dry and I'm going to give it a go.