I have a digital piano - it's 140cm long and 35cm deep, and requires a
further 5cm clearance at the back for power cables and so forth. I
estimate its weight to be between 30-40kg (it's a Roland RD800 with
weighted keys in case you're wondering!)

I have a stage stand, but I decided in my not so great wisdom it would
look nicer attched to the wall - that way it would give me more floor
space, and allow me to fit my legs under more easily.

Make a stand from wood or metal designed to give the leg-room you
need.

Do you really want to be staring at a wall every time you play?