Hot water too much flow, warm water.?

I have a Worcester 24 CDi boiler combi
The hot water is a problem, for example, if I run a bath, and put the hot tap on full, it used to restrict the flow depending on the temperature set on the front panel of the boiler.
But ohh no it doesn't, it used to, but now I get a mega blast and a bath full of warm water. The knob on the front does not change the flow rate/temperature.
I have been onto bosch about it and they recommend replacing "Domestic Hot water thermister part number 8716142302" Now I am not an expert, but this I believe is the component strapped to the hot water exit pipe, it has some heat transfer compound and two yellow wires connected to it. I have taken it off and had a look, and reconnected it. Now assuming I have the correct component, what are the odds of it failing????, can I measure the resistance across is as the hot water flows and heats up the sensor/pipe.
Any advice would be most grateful
Many Thanks Phil
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Does this boiler really have some sort of valve to restrict the flow? Normally, combis don't and you have to not draw more than it can produce. It is normal to back off the isolation valves to the bath taps to prevent more water being drawn than can be heated. It would start getting noticable around this time of year because the ground temperature is starting to drop, and less water can now be heated than during the summer. Alternatively, the isolation valve was backed off and someone has just opened it up full, without realising the consequences.
Christian.
P.S. I'd suggest reposting to uk.d-i-y, where there is much combi experience. Someone there might know if the 24CDi really does have a controllable flow reducing valve as you suggest.
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Just noticed that you already have...
Christian.
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