Given that the thermal cutout is inside the machine it will be self resetting - the thermal trip may be defective and not resetting itself -or- something other than the thermal trip may be defective (cable, switch, motor).
People on E-bay sell these things for £3.40 with different suppliers charging varying amounts for postage.
dysonmedic.com has nice pictures and instructions showing how to strip and reasssemble dysons.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.