Hiya. So, you've got the switch housing off. Do you get a positive click when you press/release the switch? - I doubt very much the problem is with the switch.
Have a look at the post-motor filter. If it's full of carbon it will restrict the airflow away from the motor, causing it to overheat, therefore causing the TOC to kick in.
With TOC's, sometimes they reset themselves and sometimes they fail in the open circuit position (all it is is bi-metallic switch!).
I think you're probably going to strip your machine, pull it's motor out, do a continuity check on it's TOC.
If there is black shite everywhere and the armature is black/looks very worn, it's time to call Stuart/Olga/Claire at Manchester Vacs
as much as it pains me with me being a Leeds fan!