I have just done a complete descale of my solis to try and fix the steam valve. So I disassembled the whole machine. As the boiler is quite small I put it into a small ice cream container with it completely saturated with citric acid. As soon as the scale touches the solution it started bubbling and it was gone! I thought it would flake off as others have said before.
On a side note, I didnt remove any of the electrical components on top of the boiler as it looked quite hard to remove... Was wondering if those parts are alright if they have been in contact with water. If they are then Ill leave it to try for a few days and get a hair dryer at it.