No heat/low pressure light - limescale in heater?

My son's Triton 2 is about 5 years old and he lives in a hard-water area. After good service, the shower has suddenly stopped delivering hot water. Cold flows out the hose no matter what heat setting is selected, but the Low pressure light comes on when heat is selected, whereas it doesn't with the "Cold" button selected.

After watching your video's I checked the TCO (all Ok) and checked the water inlet filter (virtually clear). With the shower head removed from the hose, the cold selected and temperature dial at minimum to get max water flow, some dark coloured scale came out the hose (it looked a bit like metal swarf, but was actually discoloured limescale). Water flow seems good. It seems to me that maybe the heater needs descaling and I wonder if this could be restricting the flow and causing"Low Pressure" to light up? (We use "Oust" on the shower head about every 2 months as the rose holes become clogged with limescale.)

Is it a difficult job to remove the heater assembly to do this, or is it not worthwhile? Given the cost of a replacement 9.5kw heater, am I just as well getting a new shower unit complete?