Thanks for the suggestion. Difficult to get it on the nut as it would have to travel over the end piece first, which is a bigger diameter. I'm thinking I'll have to cut away more of the back board and try a spanner.
I had a new Villeroy and Boch wall hung toilet installed a couple of months ago. Last week I had a tiler come to tile the cloakroom, he removed the toilet etc and tiled the room. I have to refit the toilet and now the distance has increased by 1cm. It's fixed by a securing frame behind a stud...