I am thinking either the soft touch switch has shorted and is making a permanent live, or there is a fault on the PCB.
How practical are you? If you have a test meter and know how to strip the machine down some, I can give you some pointers. If you are not practical, this is one for the repair shop. The DC15 is not a terribly simple machine.
But can you clarify.......
I have removed the switch and switched on my hoover
What switch have you removed?