I've had my Triton T100si for 3 1/2 years and has been working fine, am no longer getting hot water and no water pressure. The water only comes out of the overflow.
This is pretty commonplace - if you're in London or its environs, and without descaled or softened water, then I'd expect your heater can to be scaled up after 3.5 years.
The remedy is to buy and fit a new heater can. Prices from Triton direct are competitive.
If you post the rating of your shower, e.g. 8.5kW, then I can tell you the part number and point you towards the relevant web page.
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