The link above should solve your problem. I have the same problem and my parents who bought the same tumble dryer also had the same problem. Its water backing up to the fan then its hitting the heating element which is tripping the electric off.
At the back of the Tumble dryer near the bottom you'll see a panel that has about 8 screws that you can taqke off as well as a small panel that cover the pivet for the drum. If you unscrew this you will see the fan and heating element and no doubt a small puddle of water and damp dust that has clung onto the panel. You should dry this with kitchen roll and clean the old dust off then carefully turn the tumble dryer on (at your own risk lol) or you can take the advice of the person who posted the link above and use a hair dryer. This will stop the element getting wet and tripping off.
Also there is a water reservoir to the right of the panel i mentioned. If you prise the clips open and remove the top section you will see the water in the reservoir. If you look inside it on the bottom left hand side there is a little **** that the water drains in from. If you use something like pipe cleaners or a knife and wiggle it about you may have some fluff or hair trapped there that is making the water back up to the fan and causing the tumble dryer to trip off.
Hopefully this willsolve your problem for the time being but the filter in the bottom of the dryer needs to be cleaned thoroughly every week or so to stop this happening again.
Personally i think this is a major design flaw and the tumble dryer should be recalled but Hotpoints aftercare service is awful to say the least so lesson learnt, don't buy a hotpoint tumble dryer!