I dont think the ice cream is causing it lol .
By looks of where that pin is that is your problem the divertor valve is in a mid position between heating and hot water so when the heating is called for it is basically circulating around the boiler and not going down the heating .
Can be caused by a leak on hot water pipe to tap (making pin move but not enough to reach pin ) easily checked turn off the cold tap under the boiler and see if the pin retracts if it does then that confirms a leak .
Other than that its pins sticking/damaged in main diaphragm housing or in divertor and while need stripping and cleaning
Try the cold water off first if pin doesnt move back in push it back with a screw driver and heating should heat up confirming fault
By looks of where that pin is that is your problem the divertor valve is in a mid position between heating and hot water so when the heating is called for it is basically circulating around the boiler and not going down the heating .
Can be caused by a leak on hot water pipe to tap (making pin move but not enough to reach pin ) easily checked turn off the cold tap under the boiler and see if the pin retracts if it does then that confirms a leak .
Other than that its pins sticking/damaged in main diaphragm housing or in divertor and while need stripping and cleaning
Try the cold water off first if pin doesnt move back in push it back with a screw driver and heating should heat up confirming fault