Totally bonkers! And yes, the engineer who replaced the PCB was in a rush to get off so didn't wait to monitor a full HW cycle. Second engineer spent more time looking at the system and relaced the thermistor 'just in case'.
Cylinder is fully insulated (as supplied) so minimal heat loss. Rapidly coming to the conclusion from everyone's comments that the cylinder should have been changed 3 years ago when the new boiler was fitted. It did not occur to me that it was necessary and neither of the local (and reputable) companies who quoted mentioned it.
Total cyclinder height approx 42" - mid-point of thermostat 12" from bottom and 6" above return connection.
Great analogy and undoubtedly correct.
Don't know after 20 minutes but after shower and washing up this morning I was still getting HW at 60C+.
I'll continue to run some checks and I'll wait to see if WB Technical Dept come back with any comments and then I will challenge the installer. They were told about the issue within a few days of completing the installation but their solutuion was just to increase the boiler thermostat setting.
Thanks again everyone.
42" high cylinder is ~ 150L??, I really can't see why that cylinder should be thrown out, why not simply install a gate valve on the cylinder coil and throttle it, this has to increase the coil dT and the boiler may cut back in "normally", heating say 120L? from 18C to 60C in 20minutes means a average coil/boiler output of 17.6kw.
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