Is there a thermocouple on this model that might have failed?
There are two possible motors fitted to DC08's. A YDK (with a number on it starting "YV") or a Panasonic. The YDK is the more popular; that has a thermocouple screwed to the outside of it that is easy to swap and not expensive.
or does it have a cut-out that means it wont operate with the casing undone?
I don't recall they have such a device.
Have you checked that you have a live at the other side of your switch bypass? It may sound a silly question, but most cylinders that are inexplicably dead have a break in the cable inside or in the rewind mechanism. Testing the live at the switch would help us eliminate the cable/rewind mechanism being at fault.
If you have a live at the switch, I would remove the motor and bench test it before doing anything else.
Blocked filters would cause the motor to cut out as you suggest, but having cooled down, it should run again. That's why I want you to check the continuity of the lead and that you are getting power first.