Myson Apollo boiler: Loss of hot water

[Kav69]

Hi folks,

I've got Myson Apollo cabinet boiler (System Boiler) and recently I loose hot water after about 5min in the shower. I have been told that this might be due to faulty 'Dip Tube' but I'm not sure where this might be! On the boiler or the tank?
I have got a little space above the boiler to the ceiling and would be very awkward to replace it if it's there. Does this mean that I have to remove the boiler from the mount on the wall?
Does anybody have diagram of this boiler?
Suggestions greatly appreciated.

Regards

Kav