I’m getting strange behaviour following installing a new light in my loft. I took at spur off the main light circuit via a new junction box into another new junction box which is connected to the new switch and light. The old lights continue to work correctly, however when the new light is turned on, if one particular light in the existing circuit is on the new light will not work. If the old light is off when the new light is switched on, initially the same existing light and the new light glow dimly before the existing light goes out and the new light lights up fully. Again if the existing light is then turned on the new light goes out. Any thoughts on what may be wrong would be appreciated.