how long has the original light been in ?..sounds like a bodge job using the wrong cable.and YES it should be earthed. Just a thought , but where are you seeing these coloured wires..inside the casing ?..maybe you are not seeing the earth as its bolted/screwed to the casing away from the main wires..
could even be its been wired to a standard ceiling rose cable....
As regards the cooker ,depends on size ..KW etc, if its of the higher rating and its on a dedicated curcuit, i would of thought a definite no,no..to a second one..cooker curcuits are not to be played with without a good working knowledge of electrics.
http://www.ultimatehandyman.co.uk/diy_electric s_cooker_circuits.htm