what amp fuse in uk power cable for 27inch iMac?

my iMac helpfully shipped with a foreign power cord, it's no biggie as I have plenty of spare kettle leads at home. except yesterday after 2weeks of use it would not turn on. I changed the cable and it's all working fine but does that mean it needs to have a 15amp fuse instead of the 3amp one the first cable I used had?

I read somewhere that the fuse recommendation depends on the power consumption of the device but I haven't found this (in watts) anywhere. I know the fuse in the plug only actually protects the cable itself and the iMac will have an internal fuse of it's own but i don't particularly want to be replacing the plug fuse every 2weeks.

cheers in advance