Hello my name is Ivor.
I hope I can point you in the right direction.
Firstly take the cover off the shower and check that the neutral (blue) wire hasnt burnt or shorted. This is a problem with some showers make sure the connections are tight . Another fault maybe that the rcd is too sensitive . Try changing this and see what happens then. as it is a new shower I would not think it could be the heating element element or thermostat.
Please come back and let me know what you find