Worcester Bosch 28i junior, hot water fault.

[Sharp Point]

Good evening guys n gals and happy new year,
I got called to the above boiler today as the customer said the hot water was running hot cold hot cold. I ran the tap and the boiler fired up and the hot water got warm but I wouldn't say hot then boiler stopped firing up, I left the hot tap open and it fired back up again a few moments later and repeated the cycle. I assumed it to be the old plate heat ex gunked up, so I isolated boiler and drained, removed phex and it did have quite a lot of scale particles, I soaked in brick acid and it came up like new.
I reassembled the boiler, recharging the expansion while I was at it and refilled system. I ran the hot tap again and nothing had changed, still got hot cold hot cold. The flow switch is operating as its energising the actuator that's moves the diverter paddle.
I'd appreciate your help as its a bit of a head scratcher. I don't want to buy any unnecessary parts.

Thank you in advance.

 

[TFGplumbing]

I'd try the DHW sensor OP, literally a 5 min job

 

[hammers4spanner]

As above had one not so long ago with hot water luke warm changed DHW sensor and worked fine