Main eco elite combi boiler - luke warm water

Evening all. While running the bath for the kids tonight the water never really got very hot. It has been fine up until now so this isn't a slow deterioration in heating but has happened since my shower this morning.

Is this most likely to be the diverter valve letting by on the boiler? While I live in a notoriously hard water area, if the heat exchanger was scaled up then this issue wouldn't presumably happen while I was out at work today!

The burner does light during hot water demand and the warm water issue is present at all hot taps in the house. Central heating not been used at all since April but I will turn it on now to see if it makes a difference and I get hotter water. If I do then I guess this also suggests faulty diverter?

Edit: Hmmm I put the heating on (this worked fine) but the hot water remains lukewarm at all hot taps. Could this be some sort of thermostat limiting the hot water temp? But if so, why is the burner light still on?

Any ideas?

Thanks

I've put a multimeter across the DHW sensor and am getting 9.8kΩ. Does this sound like the sensor needs replacing?

From what I saw online it should normally be around 11-13kΩ. Would this explain why the water is getting lukewarm and the burner is on but not heating the water enough?

Thanks

An update for anyone that bothers reading it in case it helps someone out down the line.

I replaced the DHW sensor. Pretty straightforward job to be honest and now have hot water again. The new sensor has a higher resistance than the old one I took out so it does suggest that the older one had been failing. Fingers crossed the hot water remains...