I'm installing a replacement fan in a bathroom. It's a Marley NDX100T Timer Fan. There are three wires entering the fan (yellow, red and blue). Blue is neutral and the others are both live (from the light switch). If I have the two lives one way round the fan goes off at the same time as the light (i.e. the timer does not engage). If I swap them over the fan does not go of at all. Can anybody offer a solution??