boiler wont fire

Hi all. I hope you guys can shed some light to my issue. We got a Worcester Danesmoore 20/25 oil central heating boiler. It was here 4-5 year ago when we bought our house. From day one we have constant battles with the boiler which was always boiled down to either the photocell or the control...

As my hot water is from solar I only need the 'central heating' on. But now I find the boiler will only fire up and the central heating work if the 'domestic hot water' is turned on too. It has been fine before but now when I need heating on more it refuses to fire up unless the hot water is...