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Hot water cylinder making luke warm water only.

The hot water cylinder started making luke warm water only. I checked the thermostat and it was set to 60c and had been previously making water at around this temp. Turned the thermostat up to 70, left it for the day but, still this evening water is tepid at best. Cylinder doesn't appear to be to be particularly old. Let me know what the likely problem is and how much this will cost to sort.