Vaillant igniting issue ( domestic ) not CH?

Hello to everyone,
I have a Vaillant sine 18 T-3 combi boiler that was installed in 1985. Have had parts replaced over the years but now my cover with British gas has ended i have found it will only ignite on CH demand, not when you use the hot water/shower taps. Was an intermittent fault at first. I have changed the flow switch but it's not that .
Any ideas or members with a similar problem then i'd be grateful to from you,
Allan