I did have the older 850dl but its now a 950dl with same problem.
I think the Chrome outer section shows your shower is one of the older models.
http://www.argos.co.uk/static/Product/partNumber/8329769.htm
As you can see the new one is all white.
I had the old 850dl model variant myself.
It seems the fault has carried itself over the model revamp as well.
Just cosmetic changes from what i can see.
I always keep all broken bits and pieces so i have the old shower in my garage in case i need some bits microswitches etc.
At least yours has lasted more than 2 years think yourself lucky.
If you do take the cover off the front just make sure you switch off the breaker at your main fuse box - the voltages in showers are one of the highest power draws in the house and could zap you in an instant.
I think each element in the heater is activated by each microswitch contact so it could just be a microswitch this time as they were not replaced last time it was the whole heater can i replaced and I cant see that going down the tube in 2 months.
Its a sad fact that once one bit goes the rest is soon to follow.
False economy to just replace bits its best just buying a new unit when its a shower in my experience.
Sadly the so called Creda engineer refused to fix it because he couldnt get the pipe and olive off the water entry point - took me 5 minutes so he left me all the bits - great customer service i must say - not too happy when they sent me a bill for £70 because the engineer couldnt do his job.
They are the official fixers for Creda warranty but after i complained they rescinded the £70 - terrible service.
I wont be buying another Creda thats for sure.
In order of failure i would say it is either:-
Microswitch
Main Pcb
Heater Can
Will let you know how i get on hopefully will be able to test it tonight or tomorrow morning.