A few thoughts, all gas boilers fire up at relatively high rates, (I think) up to ~ 70% of output but should then modulate down very rapidly, they generally fire up at the SP-5C and will cut out at SP+5C so if the demand is < 5.5kw (your boiler minimum) the boiler will cycle on/off but not like shown in your post. One of the reasons I asked you to drain down your cylinder until cold is that the coil requirement should ensure that the boiler will run continuously for 20 minutes if everything is "normal" and then start cycling.
Another test (with CH only on) would be to shut off all rads except 1 on say your downstairs circuit, turn up the room stat and see will the boiler cycle normally on this, if the boiler acts up similarly as originally, then IMO it may point to some modulation or other boiler problem.
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Just looking at your readings in post #11 I notice that the boiler fires up again ~ every 30 secs or so after cut out, Afaik all gas boilers have a anti rapid cycling time of around 3 mins and also most have a pump overrun to cool down the heat exchanger, this also lowers the return temperature so when the boiler fires up it has modulated down before it reaches its cut out temperature, one way or the other that doesn't look like normal operation to me, someone familiar with Glowworm boilers might like to comment.