Biasi boiler brainteaser

b*ll*cking bl**dy Biasi boiler brainteaser :-/
It's a 24S combi. CH works, except that the 3-position switch (ON|OFF|timed) is broken and stuck in the centre OFF position. On DHW it seems to require a lot of flow* to get the boiler to fire up at all and when it does the boiler comes on full bore for a second or so then modulates down to a much lower burn rate, resulting in lukewarm hot water.
Does the same for summer or winter position of that switch. Temperature control know is turned up to max: if it is turned down even a little the boiler stops firing.
The diverter valve seems to be working: voltage across DHW flow switch contacts changes from 12V to 0V when the HW flow rate reaches the point that the boiler fires up, and with a judiciously placed mirror (rather than dismantling the machine as per the service manual ) I can see the actuator shaft of the diverter valve moving out and back operating its microswitch as a hot tap is turned on and off.
* seems to me, but I don't know how it was when it worked OK or what other samples of this model are like.
There are two thermistors: on the first picture in the manual both are labelled 'D.h.w. temp probe'! but on the wiring diagram there's a 'Domestic hot water NTC' and 'Central heating NTC'. Both have about 8V across with nothing happening, dropping a volt or two when the boiler fires up on DHW demand.
At this point I ran out of clue. Anyone got any to offer?
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+ If a job's worth doing, it'll still be worth doing tomorrow.
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