Uk electricians Q a light is difficult to get at as flat roofed and is faulty can you spur off another light?

I know normally lighting in UK runs in series but there is a fault in the bathroom light but cannot access it in the attic or any other way to repair or renew the wiring as it goes from the bathroom to the Kitchen which is flat roofed and its a 1920's bungalow. what I am wondering can I run another Twin and earth wire from the Living room Light to the bathroom light thus creating a spur or is that illegal in uk housing or would it overload the lighting? and if it is ok how would i do it from the living room ceiling rose which has 8 terminals to the rose and also how would I run the switching in the bathroom for the light your help would be appreciated