An ordinary manual blower speed control usually uses a tapped resistor. As you switch to slower speeds more of the resistor is brought into circuit. My guess would be that the first section of the resistor has gone open-circuit.
many thanks for ur reply m8,,sounds like a good place to start ! was looking on ebay and the resistor's seam quite cheap,,have you any idea where their situated on the car and how easy they are to change?,,many thanks...sime