Thorn Olympic 38/50b Boiler Fault?

I have a Thorn Olympic 38/50B boiler system that has been installed for over twenty years. GC No 4178920.Ser No 324129.When I put the control to hot water only the whole heating system comes on. Is this caused by a controlled valve of some sort that is now faulty ? If this is the case could anyone give me an estimation of the cost involved. Obviously with a boiler of this age it will want replacing at some point so I have to keep this in mind. I would be gratefull for any help.