Do you have and have you read the user instructions for the heater? When you tell us " tripping fuse when switched on" which "fuse" is is "tripping" or do you mean the RCD or an MCB is tripping in the consumer unit? precisely which is important.
Elddis tend to use Whale water heaters( rapidheat) fitted under the front seats. and on later ones an underfloor unit. In either case its a strip down to check the heater element