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It it doesn't matter if it is a one pipe system , they could have still given independent hot water and heating.They could have added separate motorised vvalvesone for the hot water and one for the heating.
If they didn't study here at plumbing college they probably have no idea about one pipe systems .
Sorry , but the way they have left you his unacceptable .
Ask for their gas safe number
I'm sorry but it has been installed incorrectly. There's not much more I can say . I would get them back to sort it out .