hot water problems with worcester 240 boiler?

Hi

Can't seem to have hot water unless the heating is on. Being warmer now I do not need to have the heating on in the house but it's the only way I can get hot water. Once heating is on I get hot water for kitchen sink and bathroom sink but if i need a bath I also have to open the cold water tap in the sink to get it have enough hot water! Any advice on what the problem could be would be appreciated.
Kind regards

Magwen Griffiths