thelia 23 boiler. No hot water upstairs when heating is on?

I have a Saunier Duval combi boiler, Thelia 23. Hot water works fine upstairs and down when the heating isn't switched on. When it is switched on we are not getting hot water upstairs, although downstairs is fine. We did used to get hot water upstairs with the heating on, but it has got progressively worse each winter. Last year the hot water took forever to heat upstairs and then kept cutting out and coming back on. This year it is not heating up at all, so we are switching off the heating each time we want showers/baths upstairs to get our hot water. Any ideas gratefully received!