Can anyone tell me why my Creda 79353 Storage Heaters keep blowing a fuse?

Can anyone tell me why my Creda 79353 Storage Heaters keep blowing there thermal fuse.

First of all all five heaters in the house were working fine. But one stopped working so I called out an engineer to fix the problem who charged me £75 to repalce the thermal fuse. Then the problem happened a week later to the same radiator. I ordered the parts myself this time and replaced the part only for the fuse to burn out again a few days later. Then the next room the same problem happened, again I replaced the fuse and days later the thermal fuse blew. now only one radiator is working in the house.
I'm lost to what to do.
The model number is 79353 & 79343.

Can anyone suggest why this is happening. Is there something else I need to do when replacing the thermal fuse?
Any help would do???