I've got a similar problem - But I'm replacing my old system with a new WB 28i as I'm sick of running out of hot water.
At the minute the heating is on 2 circuits, termostat upstairs and downstairs, and the downstairs only comes on if it's freezing cold. Plumber had a look and it's a problem with the downstairs thermostat so thats something to check if you're on 2 circuits and also the gas pipes from the ground floor to upstairs aren't big enough (not sure if thats contributing to the problem or not) should be 22mm or something but these are quite smaller so they're getting replaced when we ger the new boiler
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